Why all the Russia love? Too much weight placed on Hitler and Napoleon I suppose...but everybody forgets Ghengis...
United States.
Worlds leader in Military technology, GDP spent on defense, and military ability put aside...it comes down to economics and domestic resources. The US has the worlds 3rd largest population, largest GDP, largest industrial base, largest domestic oil reserves outside the Middle East and second largest land area. Not to mention the fact that the average American has historically 'rose to the task' when it's leaders have called for it, to a level that makes the Russian 'Protect the Motherland!' to shame.
Russia's primary advantage is weather, something they can't even control. Their terrain advantages are simply based around the distances involved something that modern warfare has made a much less pressing issue. Rememeber that the failed invasions of Russia involved armies who were using leg infantry and wagons (Napoleon and Hitler) while the sucessful invasion and conquest was undertaken by extremely mobile forces (the Mongol Hordes). While the physical barriers in place, the bulk of Russia's population and industry is on the Western side of the Urals, the main barrier that Russia possesses on land.
America's natural barriers involve two major oceans, and two massive mountain chains, plus a major north-south river dividing the country into several portions. The east coast stretching from Boston to Atlanta is more or less urbanized and semi-urbanized terrain interspersed with forests, the West coast boasts numerous mountain ranges interspersed with thick forests, deserts and heavily urbanized areas. The southern route through the Gulf Coast has the disadvantage of being swampland and thick forest, while most major seaports and coastal zones are urbanized. None of that is condusive to modern mechanized warfare. Take the tanks, APC's and Trucks out of the equation and America makes Russia look like a walk in the park. You need to get over the Rockies or the Appalachians before you can even get to those placid rolling fields that are so lovely for tanks.
IF we're looking at Nations who are simply cost prohibitive to invade, don't think Russia. Russia has beaten off invasions, they've never really DISCOURAGED invaders. Two German invasions, a French invasion, multiple Chinese invasions, a Mongol invasion (which succeeded)not exactly a stellar track record for Russian National Defense...especially when most armies like to discourage wars before they start.
They can defeat invaders, but the invaders keep coming. Switzerland beats Russia when it comes down to discouraging invaders from even trying, Great Britain managed to do more to defeat German invasion than the Russians did. Invading America filled the professional soldiers who realistically considered it (German High Command in the early 1900's) with terror. Russia? Russia just keeps sitting there and their enemies keep thinking they aren't so tough, no matter how many times they survive invaders.
