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Given that the US military exists, what should be the limitations, if any, on its use?

Defending US territory (& considering U.S. possessions, that covers a lot more territory than most people visualize) and, in concert with international forces, humanitarian concerns. I like Churchill's response when asked why Britain was opposed to Germany in WWI & WWII when they'd been allies against Napoleon. He said Britain will join in the defense of attacked nations against attacker nations, no matter who they are. (That's why the Brits had a fit when they had to declare war against Finland, who'd been attacked by U.S.S.R., when they joined alliance with U.S.S.R. after the latter were attacked by Germany.) It doesn't always play out in reality, but it is a great precept to at least try and live up to.