I realize the US military is much stronger per capita, but in a hypothetical contest, what's the chance of defeat vs. 120 million infantry in a militia?
It's not a question of "defeat". It's a question of control, whether that be control of infrastructure, control of territory, etc., and the advantage there lies with professional militaries.
Properly supplied and constructed, lines of control can be established that are nearly impregnable against the kind of assault a militia could hope to muster, especially considering that they won't have close air support like the military will.
More to the point, even during the American Revolutionary War itself a great proportion of the people had better things to do than join the militia or the Continental Army to overthrow the British. Many were avowed patriots. Some were Loyalists who were willing to fight for King and country. But most just wanted to get on with their lives and didn't do much to support or harass either side, and the effect would be even worse nowadays.
I mean if you could get 120 million militia in one spot to attack somewhere, that place would likely fall if only due to running out of ammunition. But 120 million people in one spot, fighting a military, is something that would not long last once the artillery and bombers are vectored in.
