I wanted to debunk those.. anti-American or Canadians who often goes. "AMERICA WILL FALL!"
First of all, the chance of the United States being invaded by a country is.. well impossible or just outright stupid, here is why.
Invasion force:
The population of the United States is around 300+ million people, they have one of the largest population in the world.
Your army have to out fit over 150 million or more Americans and attack each states as every states have it own military.
You need to hold back the Navy and the Navy is the largest and more advance in the world. According to many sites, the United States own more aircraft carriers than all of the countries combine.
The People:
According to sites, the United States have more guns than all of the world military stockpiles combine. Around 80 million guns are made per year and are sold, own, and stored in the United States.
According to the U.S. Constitution, the people are automatically a member of a militia, you can take arms against the government if the government isn't allowing freedom or when a country is being invaded.
Rally:
In time of crisis, the Americans tend to 'get together' and focus their attention on the problem at hands. This was proven in both world wars as they was attack of threaten by another major power.
Weaponry/Defense:
America put more money into defense and second in the military (Israel is the first).
America owns more planes and anti-aircraft than most countries.
The Land:
America even with 300+ million people is still not overpopulated, when the invading army comes in, the states are the biggest risk. Every time you invade a state, you face a flank or a counterattack by another state and unless you have a massive manpower, the chance of your armies surviving is very little.
That's all I got so far. :/