No matter how many people you kill, using a machine gun in battle is not a war crime because it does not cause unnecessary suffering; it simply performs its job horrifyingly well.
What is and isn't justified by military necessity is, naturally, open to interpretation. One of the key concepts, though, is the law of proportionality. A military attack that results in civilian casualties - 'collateral damage' - is acceptable as long as the military benefits outweigh the price that is paid by humanity.
If you shell a military base and happen to kill civilians, you have not committed a war crime; if you deliberately target cities and towns, you have.
The American military generally counts on a kill ratio of 10 to 1 when fighting lightly armed insurgents: for every dead American, there are probably 10 dead enemy.
No one will remember that President Obama supported the Arab Spring if it eventually fails and the region collapses back into the political Dark Ages. If we actively engage these movements with advice, with money, and, when necessary, with military force, then we get a vote in how it all turns out.
