A wonderfully researched new book, six years in the works, answers these questions in a manner you'll find engaging whether you've ever asked them or not. It's called Base Nation: How U.S. Military Bases Harm America and the World, by David Vine.
Some 800 bases with hundreds of thousands of troops in some 70 nations, plus all kinds of other "trainers" and "non-permanent" exercises that last indefinitely, maintain an ongoing U.S. military presence around the world for a price tag of at least $100 billion a year.
Why they do this is a harder question to answer.
Even if you think there is some reason to be able to quickly deploy thousands of U.S. troops to any spot on earth, airplanes now make that as easily done from the United States as from Korea or Japan or Germany or Italy.
It costs dramatically more to keep troops in those other countries, and while some base defenders make a case for economic philanthropy, the evidence is that local economies actually benefit little -- and suffer little when a base leaves. Neither does the U.S. economy benefit, of course. Rather, certain privileged contractors benefit, along with those politicians whose campaigns they fund. And if you think military spending is unaccountable at home, you should check out bases abroad where it's none too rare to have security guards employed purely to guard cooks whose sole job is to feed the security guards. The military has a term for any common SNAFU, and the term for this one is "self-licking ice cream."
The bases, in many cases, generate an enormous amount of popular resentment and hatred, serving as motivations for attacks on the bases themselves or elsewhere -- famously including the attacks of September 11, 2001.
Bases around the borders of Russia and China are generating new hostility and arms races, and even proposals by Russia and China to open foreign bases of their own. Currently all non-U.S. foreign bases in the world total no more than 30, with most of those belonging to close U.S. allies, and not a single one of them being in or anywhere near the United States, which would of course be considered an outrage.
Many U.S. bases are hosted by brutal dictatorships. An academic study has identified a strong U.S. tendency to defend dictatorships where the United States has bases. A glance at a newspaper will tell you the same. Crimes in Bahrain are not equal to crimes in Iran. In fact, when brutal and undemocratic governments currently hosting U.S. bases (in, for example, Honduras, Aruba, Curaçao, Mauritania, Liberia, Niger, Burkina Faso, Central African Republic, Chad, Egypt, Mozambique, Burundi, Kenya, Uganda, Ethiopia, Djibouti, Yemen, Qatar, Oman, UAE, Bahrain, Saudi Arabia, Kuwait, Jordan, Israel, Turkey, Georgia, Afghanistan, Pakistan, Thailand, Cambodia, or Singapore) are protested, there is a pattern of increased U.S. support for the government, which makes eviction of the U.S. bases all the more likely should the government fall, which fuels a vicious cycle that increases popular resentment of the U.S. government. The U.S. began building new bases in Honduras shortly after the 2009 coup.
Vine also tells a troubling story of the U.S. military's alliance with the Camorra (the mafia) in Naples, Italy, a relationship that has lasted from World War II to the present, and which fueled the rise of the Camorra -- a group reportedly deemed reliable enough by the U.S. military to protect nuclear weapons.
The smaller bases that don't house tens of thousands of troops, but secretive death squads or drones, have a tendency to make wars more likely. The drone war on Yemen that was labeled a success by President Obama last year has helped fuel a larger war.
In fact, I want to quibble with Vine's account of the birth of Base Nation, because I think the facilitation of the worst war ever was involved. Vine gives the history of the U.S. bases in Native American lands, starting in 1785 and very much alive today in the language of U.S. troops abroad in "Indian territory." But then Vine dates the birth of the modern base empire to September 2, 1940, when President Franklin Roosevelt traded Britain old ships in exchange for various Caribbean, Bermudan, and Canadian bases to be used in or after the war he was supposedly not planning on. But I'd like to back the clock up a little.
When FDR visited Pearl Harbor (not actually part of the United States) on July 28, 1934, the Japanese military expressed apprehension. General Kunishiga Tanaka wrote in the Japan Advertiser, objecting to the build-up of the American fleet and the creation of additional bases in Alaska and the Aleutian Islands (also not part of the United States): "Such insolent behavior makes us most suspicious. It makes us think a major disturbance is purposely being encouraged in the Pacific. This is greatly regretted."
Then, in March 1935, Roosevelt bestowed Wake Island on the U.S. Navy and gave Pan Am Airways a permit to build runways on Wake Island, Midway Island, and Guam. Japanese military commanders announced that they were disturbed and viewed these runways as a threat. So did peace activists in the United States. By the next month, Roosevelt had planned war games and maneuvers near the Aleutian Islands and Midway Island. By the following month, peace activists were marching in New York advocating friendship with Japan. Norman Thomas wrote in 1935: "The Man from Mars who saw how men suffered in the last war and how frantically they are preparing for the next war, which they know will be worse, would come to the conclusion that he was looking at the denizens of a lunatic asylum." The Japanese attacked Wake Island four days after attacking Pearl Harbor.
In any case, Vine points to the uniqueness of World War II as a war that has never been ended, even after the Cold War was said to have ended. Why have the troops never come home? Why have they continued to spread their forts into "Indian Territory," until the U.S. has more foreign bases than any other empire in history, even as the era of conquering territory has ended, even as a significant segment of the population has ceased thinking of "Indians" and other foreigners as subhuman beasts without rights worthy of respecting?
One reason, well-documented by Vine, is the same reason that the huge U.S. base at Guantanamo, Cuba, is used to imprison people without trials. By preparing for wars in foreign locations, the U.S. is often able to evade all kinds of legal restrictions -- including on labor and the environment, not to mention prostitution. GIs occupying Germany referred to rape as "liberating a blonde," and the sexual disaster area surrounding U.S. bases has continued to this day, despite the decision in 1945 to start sending families to live with soldiers -- a policy that now includes shipping each soldier's entire worldly possessions including automobiles around the world with them, not to mention providing single-payer healthcare and twice the spending on schooling as the national average back home. Prostitutes serving U.S. bases in South Korea and elsewhere are often virtually slaves. The Philippines, which has had U.S. "help" as long as anyone, provides the most contractor staff for U.S. bases, cooking , cleaning, and everything else -- as well as likely the most prostitutes imported to other countries, like South Korea.