In my army years I received two very different briefings about this.
The first was in ROTC before the My Lai massacre was revealed. I was told that all orders were to be followed to the letter. If I felt an order was illegal, I could protest in writing -- but only after that order had been carried out to the letter. It was up to higher headquarters to determine what was illegal, not me.
My second briefing was at the Signal Officers Basic Course at Fort Gordon in 1971, after My Lai was revealed. The JAG presentation said we were not to follow illegal orders, but as far as how to determine whether an order was illegal or not, I received a lot of weasel words from military lawyers. As a result, I left that class more confused than when I came in.
I don't know what the rules are now, but I don't see people questioning orders or disobeying them.
I hope Godly men and women will turn against those who are willing to willy nilly mowed down citizens.
I wouldn't even want the leftist anti-war protesters shot up. Those scum deserve a good beating, and sent home (or deported) to mommy, but lethal force should be a very difficult and thought out decision on our militaries part.
That said, I don't think after the first mowed down, it would be nearly as easily. We have a well armed citizenry and a lot of pretty good riflemen. It would dissolve into uncontrolled chaos and the UN and our enemies would lick their chops and move into to grab land.
I just hope the ones responsible for our nearing bankruptcy, overspending and unconstitutional maneuvers don't see the light of the other side of what they are pushing us toward.