A well armed citizenry is an extremely effective deterrent to an invading force. It's one thing to fight an air/sea war and attempt to bring your enemy to the point of surrender which is what Japan tried with Pearl Harbor. But it is totally different to be an invading force with the intent of occupying foreign territory.

History has shown the effectiveness of armed citizens against an occupying force many times. The most recent example is still going on today in Iraq. The war may have started out with the US fighting the Iraq army until they surrendered but who are we fighting now? Who is responsible for killing hundreds and thousands of our soldiers in Iraq over the past several years? Not the "organized, effective military" of Iraq but very small groups of armed citizens/insurgents that are conducting very effective guerilla warfare against the military trying to occupy their land. And how effective are these armed citizens? Well right now they have the elected politicians of the most powerful country in the world on the verge of voting to turn tail and run from them.

So is the US armed population a deterrent to an invading army? Absolutely! Imagine a foreign army invading my state of Alabama...sure the citizens alone could not stop a well equipped army but once they had won the initial battle and moved in to occupy the state the armed citizens that were left could and would wage a guerilla war against the occupiers that would lead them to reconsider if it is worth it just as the US is doing now in Iraq.

Vietnam is another example of the effectiveness of armed citizens and look at the result of Russia invading Afganistan. Armed citizens have routinely changed the course of war long after their organized, effective military has failed them.

A military leader that would fail to take into account the armed domestic population of a territory that he/she intended to invade would be making a catastrophic mistake.