1) Ignore casualties, or prioritize "your" side's body count How hard is that? Well, in terms of mainstream media reaction to the ongoing Palestinian deaths in Gaza, the biggest surprise might be that news outlets seem have been shamed into noticing the disparity at last. But Israeli soldiers', (and always American soldiers') lives remain, by certain telling, much more important than that of the civilians in the countries they invade or occupy. Some animals are definitely more equal than others.
2) Blame the other side's government – as if that removes all moral issues from the action of yours. An easy way to do this is to make everyone from Saddam Hussein to Vladimir Putin into Hitler. Being a power-gathering creep (what leader isn't?), or even a violent dictator isn't enough, the enemy of all that is good needs to be Hitler to make all actions of the good guys acceptable.