It's not the US's responsibility or right to be the policeman of the world. [...]
I certainly agree that Iran has been a debacle, and Trump decision to "do it differently" than all of the (supposedly stupid) presidents before him obviously came to bite him ass last night. This war has cost us lives, giant amounts of money, our standing in the middle east, and critical weapons supplies--and the Iranian regime is stronger than they have ever been.
HOWEVER, let me react to that comment above on the abstract, and flip it around: it is
drastically more expensive to the world when every single country needs to have an army capable of defending itself against every other army. In other words, if every country in the world had to spend what the US, Russia or China spends on defense, then we would (and may) have a much poorer world. And realistically, in this scenario, there will be lot more wars--which are also very expensive.
Yes, the USA has been bad in the last 80 years in various ways, and it has caused a lot of problems. But the world can get 10 times WORSE very easily. People living today don't remember WWII when 85 million humans perished in about 6 years. That's 2-3 times as many that died in wars in the 80 years after.
With today's technology, a very angry small country that arms themselves with cheap drones can credibly attack another country far beyond their borders and have a credible war with them, having a decent chance of winning if they sufficiently build up their arms. This hasn't been done in past mainly because of the international community--which from a military perspective is mostly the USA--would credibly threaten said country with retaliation.
Now imagine we took that, "not police force of the world" seriously. Then we let any country attack any other country and don't get involved (btw, economic and diplomatic sanctions are part of being "the police" as well). No, since we're going to take that seriously, the international response to Chili attacking Paraguay is... nothing. Let them fight it out. Or Chili attacking Columbia, or Columbia attacking Mexico, etc. etc. etc.
The military, like the police, is a waste of money. They don't do anything positive for us, only protect us (in the best case scenario) against negative things. Having lots and lots more military means lots more waste. (Obviously the other extreme is not valid either, but I am only explaining why a balance--and the post-WWII balance that lasted for 80 years--is more efficient).