

Well, taking a look at the map of the world, the US has 6,000 miles of ocean to the West, 3,500 miles to the East, a mountainous arid border with an undeveloped country to the South, and a frigid border with a small country to the North.
Canada could not invade if they wanted to. Their population is a 9th of ours, their economic output is a 10th of ours, our economies are too intertwinned to risk it, and the natural ostacles are extreme(the extreme cold, the mountains to the west, the Great Lakes to the East) . If Mexico tried(cue illegal immigration joke) I can't imagine that they would have much success either. They are an undeveloped country with a fairly small economy, a much smaller population than us, an economy heavily dependent on ours, and nearly our entire border is covered by mountains.
Cuba is far too small and poor to do much of anything in any case.
A strong Navy and Air Force can keep virtually any country away from the US mainland. Even if any countries managed to somehow gain a foothold(almost impossible), the US could create an army by the time they have arrived. Any enemy advance would be stalled, as there would be "a gun behind every blade of grass" to quote Yamamoto; the Second Amendment protecting the people's natural right to bear arms is the ultimate "standing army". This would make any occuptation by a foreign army virtually impossible. Any foreign armies would get bogged down and an American army could then be created to defeat the enemy.
Of course, nuclear weapons and MAD make any wars with major powers unlikely.
For these reasons, I view a standing army(especially a government army...) as unneeded. A decent Navy and Air Force, the Second Amendment, and the state national guards is all that is needed to protect the United States.
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