George Friedman explains the world set up with geography. The reason the US has maintained sovereignty/never been invaded (in modern times) is that it has two giant oceans on either side, a friendly neighbor to the north and a sofar ineffective one to the south. Thus America has enjoyed a geographic privilege similar to Great Britain making it hard to invade (lowers your cost of defense considerably so you can focus your military on offense) but with even bigger moats wide enough to also prevent air attacks.
As we all know the US has been constantly engaged in war somewhere on the globe for the past 100 years; something which most Americans don't notice in their daily life (there's rarely a draft and never any material hardships). If you ask the average American how many countries the US has military bases in around the world, I bet the average answer will be off by two orders of magnitude
However, aside from a brief and failed foray into nation building with Project New American Century (Bush2 administration), the general geopolitical dea to preserve US hegemony has been to destabilize regions just enough so that no real competitor can arise and challenge the US. This explains the US's rationally weird alliances in the ME. Key here is to prevent Iran (an otherwise natural ally, at least relative to the other ME oilstates) from getting powerful enough to fiddle with the oil markets which are vital to US stability. Same with the coups and countercoups in the Ukraine. Of course, the US doesn't exclusively pursue geopolitical ends. There's also the recurrent humanitarian mission (Kosovo, Somalia).
As such the military effort serves two purposes. Domestically, it's a giant stimulus program generating hundreds of thousands (millions) of jobs. Tons of pork projects. Military bases. Defense sector stock market returns. One might see this as an alternative to a welfare payment ... or an alternative to dumping interest rates. Basically, it creates jobs---like the super-expensive JSF that everybody(*) hates from a practical point of view---that don't directly produce value in people's lives but does provide them with a paycheck so they can buy this value: cars, houses, computers, fancy dinners, ... The international purpose is given above.
(*) I haven't been specifically looking, but I don't recall ever seeing anyone actually liking that aircraft which is a shame.
The "America First" policy is changing this (slowly). With the US trying to pull out, it allows regions to stabilize and the other players, China and Russia, to bring these regions which are geographically nearer and more relevant to them into their orbit. Putin is exerting control over Syria and Ukraine which allows Russia to control the energy situation in Europe (instead of the US which otherwise saw it as an outlet to stabilize its domestic shale gas and reduce the price volatility---if you can export surplus, you've eliminated much of the planning problem). China is making noise in the South China Sea (which is to them what the Carribbean is to the US) essentially trying to (re)establish their own Monroe Doctrine... similar to what the US did to Europe when the Brits began to lose power over their colonies. To some degree this is returning to the Cold War but with 3 instead of 2 players. The regional destabilization will continue because it's very effective (and very clever).
PS: I've also noticed that demopublicans are about equally in favor of this brand of geopolitics---and why not; it keeps wars small albeit constant while maintaining a global market where the US can do business buying resources and selling high-tech, treasury bonds, and (ironically) weapons. However, their means are different; perhaps favoring their constituents? Reds like good old-fashioned invasions with boots on the ground, thus providing lots of employment to the red states which provide most of the man-power for the military. Blues like cruise missiles (now drones) and special force operations, perhaps favoring the blue states which produce most of the tech-oriented GDP. Or maybe that's just a coincidence.
