A big part of why the United States gets involved with global conflicts has to with the United Nations and NATO.

In short countries make pacts between political powers to help other countries. Many can argue that it helps America keep a foothold in regions where we have interest. As an example, Syria. We have no interest in Syria but we are supporting rebels to over throw a murderious dictator who is using chemical weapons on its civilians. Who is going to "punish" the United States for helping citizens not get gassed with chemical weapons?

It's actually very simple to understand when you think of the wars we have been involved with. WW1 and WW2 we didn't even get involved in a military aspect until we were attacked. We helped with supplies and ships but we didn't fight in those wars until we we were dragged into them.

Vietnam was a French war and they needed help. Once we showed up France packed their bags and left us to fight a war that wasn't ours.

Korean War.....This can hotly be debated of the why we get involved but overall it was to reduce the spread of communism and fight Russia without fighting Russia.

The first Iraqi war was a result of Iraq invading Kuwait and stealing oil, gold and murdering civilians. Kuwait at the time and still is on of our strongest allies in the Middle East.

In a nut shell that's how the US gets away with it. Other countries depend on the support America brings to their countries.