It’s us who gets involved because it’s us who have the capacity to get involved. Of course, in fact, we don’t always get involved; Rwanda, for example, where no one got involved and most people died in a miserable civil war; Sierra Leone, where the UN tried to get involved without our help but couldn’t stop the nasty, brutish civil war; etc.
We made ourselves the world police by working to be the strongest, best armed nation in the world. With great power comes great opportunity to step in and prevent suffering. Given that most people consider suffering a bad thing, and that a small suffering for us to prevent a large suffering for others is a good thing to do (y’know, like that entire Christian thing?), it falls to us and to the other strong nations (like the NATO countries, who worked just as hard, if not harder, and suffered more in Kosovo) to stop it.
But we could’ve just as easily staved off fighting in Germany and eventually signed a peace treaty with them to avoid American bloodshed; after all, they weren’t the ones who had bombed Pearl Harbor. They hadn’t really been messing with us at all. So why go invade them and kill off thousands of American boys just to go fight someone who wasn’t a direct threat to us?
A.) Uh-huh. And then every other country decides, “Hey, if we don’t like the President of this other country, it’s perfectly acceptable for us to hire people to assassinate him; after all, they set the precedent with Hitler!” Not a good precedent to set, unless you like U.S. Presidents getting killed every three months.
B.) And that would have changed things, how? Remember, Hitler didn’t appear in a vacuum. His popularity was because many regular Germans agreed with his positions. He wasn’t a brilliant bureaucrat or strategist; he was just a good orator and poltician. But assassinate him, and what happens? Maybe you get a better bureaucrat or strategist, one who decides not to invade Russia without a better timeline, or one who truly gets German industry ramped up to production. Sure, the Nazis have lost a great orator, but now they have a martyr, someone who was ruthlessly murdered by the British/American/French/whatever government. Watch German public opinion of the Nazis jump through the roof, just like American public opinion of Kennedy (also a great speaker but not a great leader).