There are Americans who are sick and tired of the United States playing "policeman to the world." There's good news and bad news for these people. The good news is that the U.S. isn't actually the world's policeman. The bad news is that they're actually the world's policeman, fire department, emergency medical technicians, doctors, nurses, and any other global-scale first responder analogy you can think of.
The U.S. military is basically the Avengers.
While the United States doesn't respond to every trouble spot on the planet they sure respond to a lot of them. Of the 195 officially recognized countries in the world, the United States has military members deployed to 150. So if there is a trouble spot, there's a very good chance that U.S. troops could go handle a large percentage of them. Luckily, Earth's mightiest heroes are usually reserved for bigger problems, like keeping North Korea in check, punishing ISIS, and trying to bring food to hungry people.
But some of those countries are actually protected by the United States military, even if that protection isn't specifically promised. For example, the U.S. military has long been considered a pillar of Saudi Arabia's stability, because Saudi Arabia's military can't invade and win against a much-smaller neighbor, even when 20 other countries are helping them.
Seriously, the Salvation Army could have invaded Yemen and won by now.
But despite how terrible the Saudis are at things like strategy, tactics, and planning, they will never have to worry about being overcome by Iranian interference or military force because they have a substantial force they can rely on to protect their homefront: the United States military. And they aren't alone.
Treaty obligations tie the U.S. to come to the defense of 67 different countries around the world, going well beyond the 29-member NATO alliance. The U.S. has bilateral defense agreements with six different countries, as well as every individual member of the Organization of American States and the ANZUS agreement.
While the United States is no longer required to defend New Zealand and West Germany doesn't exist as West Germany anymore, the United States military still has a pretty big job on its hands. And even though relations with some of the members of the Organization of American States aren't so hot with the U.S. right now, it's still a way for Americans to find themselves fighting alongside the likes of Nicolas Maduro of Venezuela or helping defend countries with no military at all, like Costa Rica, Panama, or Haiti.
It might be worth noting that our Venezuelan allies have asked Russia to help with whatever it is they're planning to do down there, rather than ask the United States. But along with Venezuela, the U.S. has promised to defend a full one-quarter of all the humans on the planet.
That's a big job.