We showed that when Japan bombed the, wait for it, the, the, the, Phillippines. They occupied our soil for 3 years, costing 500,000+ lives, and we did nothing.
"Our" soil's a bit of a stretch, but good point.
To be real about it, those were "the little people" (those in protectorates, grabbed or conquered territories, those subjected to treaty-arranged control, etc). Indeed, the U.S., like most other similar-thinking imperialist nations, has a fairly ugly record when it comes to such people and their lands. Aside from the U.S.'s WWII handling of Europe's remaining, non-commie-controlled nations, there aren't many bright spots along those lines.
China and Russia are going to present a severe issue for the world over the coming couple of decades, I suspect. If they keep pushing, at some point the world's nations will be obliged to say "no" in as forceful terms as required.