First we kept slaves. Then we displaced and massacred Indians when they didn't cooperate with Manifest Destiny. When we decided we wanted to become an imperial power, we manufactured a war with Spain, promising the Filipinos independence if they helped us. After we reneged on our promise to our "little brown brothers" and they fought back, we massacred them, too. Next on the agenda: firebombing German and Japanese civilians. Then when the Japanese still didn't say uncle we nuked them and would have kept nuking them until we turned the entire county in a cinder pile. And we weren't satisfied with winning. We had to make an example of the Germans, so before there was a Marshall Plan there was a Morgenthau Plan that did an excellent job killing more Germans with starvation. Then toss in a few assorted CIA-plotted assassinations/coups over the decades and I have to wonder if there ever was a point in our history when we lived up to our own principles and the rule of law. Even now, liberals rationalize this president carrying out extradjudicial killings with missiles that kill every man, woman, and child in the vicinity but if they're captured we're supposed to read them their Mirandas and give them a lawyer. We sign treaties on the treatment of civilians in wartime but no one should be under the illusion that we won't unleash hell on earth and decimate the cities of any country which likewise threatens us with nuclear weapons. Meanwhile, we're still the only country that's ever used them in anger.
So I'd say we're still a work in progress and we can improve our record, but if our history has proven anything there's one principle that trumps all of the others: If you're ever in a war, even one you create, just make sure you don't lose.