Being surrounded by the mountain range has helped Switzerland to stave off invaders for ages.
National Redoubt (Switzerland) is a defensive plan adopted by the country in the 19th century that uses the Alps as a roadblock to any invasion. The plan allowed for fortification of the Alpine region and this plan was continued post-WW2 as the threat of a Soviet invasion was high.
The Swiss military has wired the entire mountains and infrastructure, such as roads and bridges, to blow up in case of any invasion so the invading party will have to waste time and energy in finding proper entry routes into the country.Additionally, many tunnels, bunkers and fortresses have been built inside the mountains that are able to house a complete division of the army, thus acting as suitable points from where the military can efficiently bleed the invaders dry through a battle of attrition.
Even though Switzerland has about 150,000 active military personnel, if the nation gets invaded almost 4,000,000 citizens are immediately available for military service with another 3,000,000 fit for service as well. Basically, in a heartbeat, the nation gets all of its able-bodied citizens ready to pick a fight with the invaders .
New Zealand is almost 1,000 miles from the nearest major landmass of Australia, which in turn is not all that close to Asia or anywhere else. So the logistics of getting troops and supplies there would be heinous. Australia is a good candidate for similar reason, less isolated but with a much larger landmass and a harsher climate. Starting the invasion of Australia would be easier, but completing it would be harder.
The prize for inaccessible island nations goes to Iceland. Not only is it in the middle of the North Atlantic, it also has no large seaports. Plus, Iceland's terrain is filled with mountains and lava fields and glaciers.
As history has proved, that would be Russia. It's territory size alone would make an invasion difficult – consider all those logistics chains you'd have to maintain. Add to that the climate which ranges from very hot in summer to damn freezing in winter with periods of enormous slush inbetween. And don't forget the lay of the land, which includes mountains, vast forests, huge swamps and lots of rivers.
And then there are the Russians, who have been fighting wars and engaging in guerrilla warfare for thousands of years. No army is large enough to keep the whole of Russian territory under control.
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