【各国人民QA回答】在地理学上，哪个国家最难被入侵？ [美国媒体]

Which country is the most difficult to invade geographically?

在地理学上，哪个国家最难被入侵？


1.Bas Leijser, Writer, mountaineer, coffee addict, student

作家，登山家，咖啡迷，学生

The Netherlands!
Yes, you read that right.
Our army is in a rather sad state right now:
1）60,000 active personnel
2）5000 reserves
3）A whopping 16 tanks (beat that Russia!)
4）Yearly budget of 8 billion (1.33% of USA’s budget)

荷兰!
是的，你没看错。
我们的军队现在正处于一个令人悲伤的状态:
1)60000现役军人；
2)5000后备军；
3)高达16辆坦克(干TM的俄罗斯)；（译者：俄罗斯坦克数量20000+，世界第一）
4)80亿年度国防预算(美国国防预算的1.33%)。

We were once the most powerful country in the world (and, sadly, one of the leading countries in slavery and colonialism) but now we are just a cute little dot on the map.
We had close to 1000 tanks in 1990 (the Leopard 2) but the government has sold all of them ever since.
And still, I dare say that our country is the most difficult to invade.
Perhaps I really should not be giving away our secret, but here it is. This is what would happen if, say, Russia would decide to invade us (purely hypothetical, might as well be USA or China).

我们曾经是世界上最强大的国家(可悲的是，它是奴隶制和殖民主义的主要国家之一)，但现在我们只是地图上的一个可爱的小点。
1990年，我们还有近1000辆坦克(豹2)，但那之后，政府就把所有的坦克都卖掉了！
然而，我敢说，我们的国家是最难入侵的国家。
也许我真的不应该泄露我们的秘密，但如果说俄罗斯决定入侵我们，以下这些就会发生(纯属假设，也可能是美国或中国)。

“Da, komendant! CYKA BLYAT!”
Several thousand tanks roll into the Netherlands all across the eastern border. There is little resistance.
Within hours, half of the country is occupied. Our 16 tanks are nothing but wrecks.
The enemies push on, The only thing that gives them pause, is the heroic counter-attack by the 34th Bicycle Cavalry Force.

“是，指挥官！CNM!”(毛子语)
数千辆俄罗斯坦克在东部边境进入荷兰。几乎没有遭遇任何抵抗。
几个小时之内，这个国家就被占领了一半。我们的16辆坦克化为残骸。
敌人继续前进，唯一能让他们暂停脚步的是第34自行车骑兵团的英勇反击。

As the enemies near the capital of Amsterdam, the following conversation takes place in our army’s HQ:
General 1: “The Netherlands is doomed! Perhaps we should have learned our lesson when we cut our army budget right before WW2.”
General 2: “We have no choice now.”
General 1: “Yes, we proceed with the plan.”
Both generals: “Activate Project Atlantis.”
The following footage is highly classified, but let it serve as a warning to any potential invaders.
Initiation of Project Atlantis would look a little like this:

当敌人抵近首都阿姆斯特丹附近的时候，下面的对话将会发生在我们军队的总部:
将军1:“荷兰注定要灭亡！也许我们应该在二战前削减军队预算的时候就吸取教训了。”
将军2:“我们现在别无选择。”
将军1:“是的，我们只能执行那个计划。”
两位将军:“激活亚特兰蒂斯计划”。
下面的视频是高度机密的，但是我把它放在这里作为对任何潜在入侵者的警告。
“亚特兰蒂斯”计划的启动看起来有点像这样（译者：然而我并不知道怎么上传一个gif文件，大家充分发挥哈想象）。

Leading to:

这会导致：

And:

以及：

You may take our lives, but you will never take our homeland!
Since the Netherlands is the only country worldwide that is almost entirely below sea-level, it even has a plan where flooding the country is used as a defence strategy (Hollandic Water Line - Wikipedia) and this can easily be done by destroying various flood defences; I say we are the most difficult to invade because there will be nothing to invade if we are desperate enough.

你也许会夺走我们的生命，但你永远不能夺走我们的家园！
由于荷兰是世界上唯一一个几乎完全低于海平面的国家，我们甚至制定了一个计划：淹没整个国家以作为防御措施(荷兰的水位——维基百科)，这很容易就可以通过摧毁各种洪水防御系统来完成。我们是最难入侵的，因为如果我们足够绝望的话，就没有什么可以入侵的了。

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2.Balaji Viswanathan, student of world history

学习世界历史

Japan. There has never been a foreign born person ruling the country in its history. Only the most powerful weapon ever - Atomic bomb - could make them surrender and even that was not enough for a full transfer of control. Even the Mongols failed there twice. Every other civilization has been invaded in the past.
Russia has been invaded [by Mongols and the Scandinavian before them]; Afghanistan has been invaded many times by Indian empires; China has been invaded [by Mongols, Manchus and Japan]; Switzerland has been invaded many times - people forget that its independence is relatively recent and as recently as in the 19th century they have been invaded by France [Helvetic Republic]; Britain had been invaded by the Romans, Angles, Saxons, Vikings, Normans...

日本。在历史上，从来没有一个外国出生的人能统治这个国家。只有最强大的武器——原子弹才能使他们投降，但就算这样也不能完全掌控它，即使是蒙古人也失败了两次。而其他每一个文明过去都被侵略过。
俄罗斯曾被蒙古人和斯堪的纳维亚人入侵;阿富汗曾多次被印度帝国侵略;中国曾被蒙古、满族和日本侵略;瑞士同样多次被入侵——人们忘记了它的独立是相对最近的，就在19世纪，它被法国入侵建立了赫尔维蒂共和国;英国曾被罗马、盎格鲁人、萨克逊人、维京人、诺曼人等入侵。

Why is Japan so hard to conquer?
Islands are always hard to conquer and Japan's islands especially Honshu is among the largest island by population. It is not easy to conquer a population that has high emphasis on sustaining itself. New Zealand is an island too, but Japan has more than 25 times its population. It is  easier to overpower the 4.4 million Kiwis than to do the same to 127 million Japanese.
Britain is an island too and has some of the advantages, but besides being a little smaller, it is far more closer to the mainland. Crossing the East China sea between Japan & China is far more harder than crossing the north sea or the English channel. Britain also has a number of powers surrounding it. In case of Japan, there is the frozen Siberia, the Korean peninsula and China being the only challenges. Korea is smaller, Russia cannot easily supply to Siberia and that leaves China as the only real threat - and they are a few hundred miles off the ocean.

为什么日本如此难以攻克?
岛屿总是难以攻克的，而日本的岛屿——尤其是本州岛，是人口最多的岛屿之一。要征服一个高度重视维持自身人口数量的岛屿是不容易的。新西兰也是一个岛屿，但日本的人口是其人口的25倍多。征服440万的新西兰人可比12700万日本人要容易得多。
英国同样是一个岛屿，拥有一些优势，但不仅规模小一些，它离大陆也更近一些。穿越日本和中国之间的东海，比穿越北海或英吉利海峡要困难得多。此外英国还有很多周边国家。在日本四周则是冰冻的西伯利亚和朝鲜半岛，中国是其唯一的挑战。韩国规模较小，俄罗斯无法轻易地向西伯利亚提供补给，而中国是唯一真正的威胁——可它们隔着几百英里的海洋。

In short, Japan is not just a massively populated island, but quite remote too.
Compare that with Canada, whose population all live close to the US border and the US could just take over all the border cities with a lot less pain than taking over Japan. It also much smaller population [1/4 of Japan].
Britain also has historical fault lines between English & Scots - an alliance that could break apart [with a future referendum] and having two powers in the island can make things easier for an outside power. Japan is far more secure in that point - there are no major fault lines between its major islands.
Island + large population + remote location + history of non-submission  = practically impossible to invade.

简而言之，日本不仅是一个人口众多的岛屿，而且相当偏远。
相比之下，加拿大的人口都生活在美国边境附近，而美国可以轻松占领所有的边境城市，比接管日本的难度要少得多。它的人口也少得多，只有日本的1/4。
英国的英格兰和苏格兰之间也存在历史上的隔阂——这个联盟可能会在未来的公投中分裂，而在岛上共存的两个权力会让外部势力更容易入侵。日本在这一点上要安全得多——在主要岛屿之间没有大的分歧。
海岛+大量人口+偏远地区+从不屈服的历史=几乎不可能被入侵。

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3.Mohit Kapila, I know quite alot about China and India

我对中国和印度很了解

I’m going to have to give this one to our rival, Australia.

我要说的是我们的竞争对手澳大利亚。

As you may know, I am a Kiwi, but even I can’t deny that Australia should be noted as quite difficult (if not impossible in our time) to invade.
Australia is, in my opinion, a country that is unenviable by any other major country. Most of the major countries are thousands of kilometres away from Australia.

正如你所知，我是一个新西兰人，但即使如此我也不能否认澳大利亚是相当难以入侵的(在目前这个时代是不可能的)。
在我看来，澳大利亚是任何其他一个大国都难以处理的国家。大多数的主要国家都离着澳大利亚几千公里呢。

Distance between:
Canberra-Washington DC: 15,962KM
Canberra-Beijing: 9018KM
Canberra-Brasilia: 14,075KM
Canberra-New Delhi: 10,365KM
Canberra-London: 17,001KM
Canberra-Paris: 16,939KM
Canberra-Moscow: 14,498KM
Canberra-Wellington: 2329KM
Canberra-Jakarta :5401km

距离——
●堪培拉-华盛顿特区:15962公里
●堪培拉-北京:9018公里
●堪培拉-巴西利亚:14075公里
●堪培拉-新德里:10365公里
●堪培拉-伦敦:17001公里
●堪培拉-巴黎16939公里
●堪培拉-莫斯科:14498公里
●堪培拉-惠灵顿:2329公里
●堪培拉-雅加达:5401公里

The point I’m trying to make is that the Australian continent is quite far away from other civilization.
A country invading Australia would need an extremely powerful navy just to get near it. The only country that could actually do real damage to Australia is Indonesia, and even they are hundreds of kilometers away. And let’s admit it, New Zealand can’t really do much to Australia.
Now that we have covered distance from other countries, let’s talk about the immense size of Australia. Australia is huge. I’m sure that even that is an understatement. Australia covers 7.692 million km2, or about 1.5% of Earth's surface!!
The climate of Australia is also quite, um … dangerous ˉ_(ツ)_/ˉ. Australia is full of some of the most deadly and poisonous flora and fauna on the planet.
And if the animals don’t get you, then the intense climate will. During WW2, the Australians drew up a line called the Brisbane line, after the nearby large city. The line was drawn to show how far the Japanese could get into Australia before the climate killed them off.

我的意思是，澳洲大陆离其他文明都很远。
一个入侵澳大利亚的国家必需要一支强大的海军来接近它。唯一可能对澳大利亚造成真正损害的国家是印度尼西亚，但即使是它也相距数百公里。此外，我们得承认新西兰对澳大利亚并没有多大的威胁。
既然我们已经谈到了澳大利亚与其他国家的距离，我们再说说澳大利亚的巨大规模吧。澳大利亚是巨大的，可能这有些轻描淡写了。具体说来，澳大利亚的面积为769.2万平方公里，约占地球表面的1.5%。
澳大利亚的风土也很,嗯…危险ˉ _(ツ)_ /ˉ。澳大利亚到处都是世界上最致命、最毒的动植物。
如果动物没有打败你，那么恶劣的气候将会杀死你。在二战期间，澳大利亚人在大城市附近画了一条叫做布里斯班线的线。这条线的绘制是为了显示日本人在恶劣气候杀死他们之前能深入澳大利亚的距离。

Australia is just a place any invader will find hard to occupy.
Edit: I just want to add that there is a sci-fi book called “Tomorrow when the war began”. It explores the idea of there being an invasion of Australia. I am, however, not saying it is at all realistic.

澳大利亚是任何侵略者都难以占领的地方。
编辑:我想补充的是，有一本科幻小说叫做《明天，战争开始了》。它探讨了入侵澳大利亚的想法。然而，不得不说这并不是太现实。

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4.Sam Montgomery, Extensive Reader

广泛的阅读者

Afghanistan, where Empires go to die.

阿富汗，帝国主义的坟场。

Before getting to my answer I would like to state that the question says "geographically" not "militarily".My answer focuses on two things:
1）The Afghani Physical Geography - Afghanistan is a mix of deadly environments.
2）The Afghani People, who represent part of Afghanistan's Geography. (Afghani work ethic)
Afghanistan is well known for it's rugged nature, and has been the center point of several wars, I am not going to be focusing on the reasons behind those conflicts but rather, what happened during the conflicts.

在给出我的答案之前，我想说的是，这个问题是“地理上的”而不是“军事上的”。我的回答主要集中在两方面:
1)阿富汗的自然地理,阿富汗是一个致命环境的混合体。
2)阿富汗的人民也代表了阿富汗地理的一部分。(阿富汗的职业道德)
阿富汗人以其崎岖的本性而闻名，它一直是几场战争的中心，我不打算关注这些冲突背后的原因，而是要看在冲突中发生了什么。

Okay so here is Afghanistan which according to Wikipedia is:
" A landlocked mountainous country with plains in the north and southwest, Afghanistan is located within South Asia and Central Asia. It is part of the US-coined Greater Middle East Muslim world, which lies between latitudes 29° N and 39° N, and longitudes 60° E and 75° E. The country's highest point is Noshaq, at 7,492 m (24,580 ft) above sea level. It has a continental climate with harsh winters in the central highlands, the glaciated northeast (around Nuristan), and the Wakhan Corridor, where the average temperature in January is below −15 °C (5 °F), and hot summers in the low-lying areas of the Sistan Basin of the southwest, the Jalalabad basin in the east, and the Turkestan plains along the Amu River in the north, where temperatures average over 35 °C (95 °F) in July."

我们看看维基百科对阿富汗的介绍：
阿富汗位于中亚和南亚，是一个内陆多山、北部和西南部均是平原的国家。它是美国创造的中东穆斯林世界的一部分，它位于纬度29° N 到 39° N，经度60° E 到75° E之间。这个国家的海拔最高点是诺沙克峰，在海拔7,492米(24,580英尺)以上。中央高地是拥有严酷冬季的大陆性气候,被冰冻覆盖的东北(在努里斯坦附近)和瓦罕走廊在1月的平均气温低于−15°C(5°F)；西南部低洼的锡斯坦盆地则拥有炎热的夏季,东贾拉拉巴德盆地及突厥斯坦平原北部阿姆河沿岸在7月的平均气温超过35°C(95°F)。”

Despite having numerous rivers and reservoirs, large parts of the country are dry. The endorheic Sistan Basin is one of the driest regions in the world. Aside from the usual rainfall, Afghanistan receives snow during the winter in the Hindu Kush and Pamir Mountains, and the melting snow in the spring season enters the rivers, lakes, and streams. However, two-thirds of the country's water flows into the neighboring countries of Iran, Pakistan, and Turkmenistan. The state needs more than US$2 billion to rehabilitate its irrigation systems so that the water is properly managed.
Alright, so I highlighted the interesting stuff, that being the diverse climate and landscape. Which means if you're planning on invading your forces better be versatile, like really versatile..Oh and to add to that, as if trekking across the country sounded bad to your troops, remember that you're dealing with an enemy who knows the terrain like the back of their hand.

尽管有许多河流和水库，但该国大部分地区都非常干旱。内陆的锡斯坦盆地是世界上最干旱的地区之一。除了通常的降雨，阿富汗在兴都库什和帕米尔山脉的冬季都有降雪，春季的融雪汇入了河流、湖泊和溪流。然而，该国三分之二的水流入了邻国伊朗、巴基斯坦和土库曼斯坦。国家需要超过20亿美元来恢复灌溉系统，以使其水资源得到充分利用。
好吧，我已经强调了一些有趣的东西，那就是多样化的气候和地形。也就是说，如果你打算入侵你阿富汗，你的部队最好是万能的，真的是万能的......哦，要补充一点，徒步穿越整个国家对你的部队来说是很糟糕的，但还要记住你是在对付一个熟悉地形的敌人，他们熟悉阿富汗就像熟悉自己的手背一样。

Yes, that's Afghanistan..

是的，这就是阿富汗......

So's that.

这也是。

And that.

以及这个。

I'm assuming you guys all get the picture, Afghanistan's climate and Geography isn't easy to overcome as an invader.
Alright now to the general population.
Afghanistan is a multiethnic society, and its historical status as a crossroads has contributed significantly to its diverse ethnic makeup. The population of the country is divided into a wide variety of ethnolinguistic groups. Because a systematic census has not been held in the nation in decades, exact figures about the size and composition of the various ethnic groups are unavailable. An approximate distribution of the ethnic groups is shown in the chart below:

我猜你们都明白了，阿富汗的气候和地理位置不容易被入侵。
好的，现在我们说说阿富汗的主要人群。
阿富汗是一个多民族的社会，它在历史上是一个交通要道，这为其多样化的民族构成做出了重要贡献。这个国家的人口被划分为各种各样的民族语言群体。因为已经几十年没有在全国范围内进行系统的人口普查了，所以关于各个民族的大小和组成的确切数字是不可得知的。下面的列表显示了各民族的大致分布情况。

Ethnic groups in Afghanistan:
Pashtun 42%
Tajik 27%
Hazara 8%
Uzbek 9%
Aimaq 4%
Turkmen 3%
Baloch 2%
Others (Pashayi, Nuristani, Arab, Brahui, Pamiri, Gurjar, etc.) 4%

阿富汗的民族分布:
普什图人 42%
塔吉克人 27%
哈扎拉人 8%
乌兹别克人 9%
艾马克人 4%
土库曼斯坦人 3%
俾路支人 2%
其他种族 (Pashayi, Nuristani, Arab, Brahui, Pamiri, Gurjar,等等) 4%。

The Afghani Armed Population is similar to other populations in central Asia, but as many nations have discovered the Afghani people are an extremely determined (although poorly disciplined) force. They're not threatening on an immediate level, but they do know the meaning of Guerrilla Warfare.
And that's the reason why no single foreign nation/empire has been able to successfully occupy Afghanistan.
Just before I end this post I would like to point out that Afghanistan isn't just about war, obviously. And that parts are pretty developed.

阿富汗的武装人员与其他中亚人相似，但许多国家已经发现阿富汗人是非常坚定的(尽管缺乏训练的)部队。他们或许不能造成很直接的威胁，但是他们知道游击战争的意义。
这就是为什么没有一个国家/帝国能够成功占领阿富汗的原因。
在我结束这篇文章之前，我想指出的是，阿富汗并不仅仅只有战争。有些地区还是相当发达的。

the city of Herat.

赫拉特市。

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5.Hoang Nghiem (严黄), Director at Stormtrooper School of Shooting at Things
On the 8th of June 1949, George Orwell’s “1984” was published for the first time.
It was a book about what the future would be like had the past turned out differently than it did in our timeline.
After a worldwide nuclear war, three totalitarian “superstates” emerged:

1949年6月8日，乔治奥威尔的《1984》首次出版。
这是一本关于未来会是怎样的书，如果过去的情况与我们的时间轴不同的话。
在世界范围内的核战争之后，三个极权主义的“超级大国”出现了:

In this timeline, the allied invasion of Normandy fails…
1）The Soviets instead defeat Nazi Germany before deciding to spread the “Workers’ Revolution” and go on to conquer all of Europe. People in the “Free World” become extra scared of Communism.
2）The US and UK ally together, America expands southwards as a safety measure (in case Japan reaches the mainland or Communism gets to South America), the crown re-acquires former British territories for the same reasons.
3）The Japanese Empire conquers most of Asia but a Chinese-Maoist Revolution overthrows and replaces it with a Communist regime (which is hostile to Soviet Communism).
4）Don’t forget that they ALL hate each other…

在这个时间轴上，盟军的诺曼底登陆失败了。
1)在决定传播“工人革命”之前，苏维埃便打败了纳粹德国，并打算继续征服整个欧洲。 “自由世界”的人们更加害怕共产主义了。
2)美国和英国的结盟。美国向南扩张作为一个安全措施(以防日本到达本土或共产主义到达南美),英国国王出于同样的原因恢复了日不落帝国的领土。
3)日本帝国征服了亚洲大部分地区但中国毛派革命推翻了它并代以共产主义政权(敌视苏联共产主义)。
4)不要忘记他们彼此都互相仇恨...…

Nuclear war *blah blah* desperation *blah blah* emergency *blah blah* hatred and thus the world of 1984 becomes reality…
According to “The Theory and Practice of Oligarchical Collectivism” (a book withinthe book):
“None of the three super-states could be definitively conquered even by the other two in combination.
They are too evenly matched, and their natural defenses are too formidable. Eurasia is protected by its vast land spaces, Oceania by the width of the Atlantic and the Pacific, Eastasia by the fecundity and industriousness of its inhabitants.”
Obviously ignoring Eastasia… there’s your answer: the hardest countries to invade are:

核战争biahblah绝望blahblah突发事件blahblah憎恨；于是，1984的世界变成了现实。
根据《寡头政治集体主义的理论和实践》(《1984》一书中出现的一本书):
“这三个超级大国中的任何一个都不能完全被其他两个国家所征服。
它们的分配太均匀了，它们的自然防御太强大了。欧亚大陆被其广阔的陆地空间所保护、大洋洲被宽阔的大西洋和太平洋分开、东亚则依靠其勤劳和多产的人民。”
显然忽略掉东亚......这是给你的答案——最难入侵的国家是：

The United States of America

美利坚合众国

The US is surrounded by 2 oceans. All of her enemies are on the other side of the Pacific and Atlantic oceans. No one will be able to invade the US successfully for centuries to come.
In fact, no one probably even has the money to build a Navy that is good enough to first beat the US Navy, and then invade America (for which you also have to get pass their bases)

美国被两大洋环绕。她所有的敌人都在太平洋和大西洋的另一边。在未来的几个世纪里，没有人能够成功地入侵美国。
事实上，没有人可能有足够的钱来建造一支足够优秀的海军来打败美国海军，然后入侵美国(你还必须通过他们的那些基地)。

The Russian Federation

俄罗斯联邦

Ignoring the crippling Russian winter, invading Russia would still be hard. It’s not as easy as you think. The US could not just invade Siberia via Alaska and move inwards to the capital. The country is vast and crosses 9 timezones, I am sure the US would either run out of food or get bored before they reached Moscow… and that’s obviously ignoring the winter as well…

就算无视俄罗斯的严冬，入侵俄罗斯仍将是艰难的。并没有你想象的那么容易。美国不可能通过阿拉斯加入侵西伯利亚，然后向首都莫斯科转移。这个国家幅员辽阔，跨越了9个时区。我敢肯定，美国人在到达莫斯科之前要么吃完了粮食，要么就会感到无聊。而这显然还没有考虑冬天的威胁。

​澳大利亚联邦

The Commonwealth of Australia

Do you remember what I said about the US? And about Russia? Apply both to Australia. A country that is essentially in the middle of nowhere but yet still huge… give up because most of Australia is desert anyway, if strategic locations were relocated there… could you really win? Would you even try?

你还记得我对美国和俄罗斯的描述吗?这些都适用于澳大利亚。这个国家基本上处于一个四周啥都没有的地方，而它还相当巨大......之所以放弃是因为澳大利亚绝大部分地区都是沙漠，如果战略位置被安置在那里……你真的能赢吗?你会尝试吗?

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6.Jon Mixon, Studied military actions from the Punic Wars to South Waziristan 

学习从普奇战争到南基奇斯坦的军事行动

New Zealand.

新西兰

Its closest neighbor is Australia, which is 1,340 miles across a pretty turbulent body of water. It has no land neighbors and any nation looking to invade would have to bring all or most of the supplies that it needs with it as the logistics lines would be incredible.

它最接近的邻居是澳大利亚，但也隔着1340英里的海洋，那是一片相当汹涌的水域。它没有陆地邻国，任何想要入侵的国家都必须将其所需的全部或大部分物资同时带进这里，因为后勤补给是难以想象的。

Besides that, both islands are exceptionally mountainous meaning that numerous ambush points can be established that would make land war very  treacherous. Finally capturing both islands and holding them without nearby bases to reinforce your troops there would make any such invasion and occupation extremely costly.

除此之外，新西兰的两个岛屿都是多山的，意味着可以建立大量的伏击点，使陆地战争变得非常危险。最后，在附近没有基地的情况下占领这两个岛屿并在这里增派军队，这样的入侵和占领将会付出巨大的代价。

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7.Jon Davis, Can find any country on Google Maps with labels turned off.

可以在谷歌地图上找到任何一个国家。

I want to say Canada. 

我想说加拿大。

The first notable reasons are these two huge oceans which basically prevents everyone in North America from concerning themselves with the threat of international invasion. Second, the entire western portion is covered with high and inhospitable mountain ranges with lower level mountains encumbering an eastern invasion. The only real way to bypass this is to sail through a gauntlet of islands and narrow straights to invade through the Hudson bay or Great Lakes. To do that would be a really dangerous operation almost surely doomed to defense forces. 

第一个显着的原因是这两片巨大的海洋，基本上阻止了北美受到国际入侵的威胁。其次，整个西部地区都覆盖着高耸和不适宜居住的山脉，而较低却连绵不绝的山脉则阻碍了东部的入侵。唯一能绕过这一问题的方法是，通过岛屿的挑战和狭窄的直道来穿过哈德逊湾或大湖区。要做到这一点是一场非常危险的行动，几乎注定要被国防部队击败。

The only best third option would be to literally walk over the North Pole, which, being that it is a frozen ocean, would be one of the most impressive logistical feats in history, followed only by one of the most difficult fought wars ever, since it would have to take place during the winter. Follow this with the fact that most the country is composed of woodlands and inhospitable tundra you have a recipe for insurgent activity that could last for years from those who know the territory well. 

唯一最佳的是第三个选择，通过北极,但这是一个冰冻的海洋,这将是历史上最令人印象深刻的后勤壮举之一,紧接着就是有史以来最艰难的战争之一,因为它会发生在冬季。另外还有一个事实，这个国家的大部分地区都是由林地和荒凉的苔原组成的，熟悉地形的人将会在此进行多年的叛乱。

I know that many would argue about that Southern border, but realistically speaking, the US and Canada have had a bromance going on for generations. Seriously, I've heard that it is the longest running uncontested border in history. While its existence somewhat seems to ignore the intent of the OP, we might as well say that there is an invisible wall protecting Canada's Southern Border in the form of millions of armed American service people and hundreds of millions of Americans themselves.

我知道很多人会争论南方的边界问题，但实际上，美国和加拿大已经有了几代人的兄弟情谊。说真的，我听说这是历史上最长的没有争议的边界。尽管它的存在似乎不符合楼主的问题，但我们不妨说，有一堵无形的墙保护着加拿大的南部边界，它以数百万美国军人和数亿美国人组成。