If you're going 1898, the thought is this - the U.S. goes for broke in the Spanish-American War. IOTL, Spain was devastated by the loss and lost a whole bunch of territory. If the U.S. runs up the score, it conquers Spain after bleeding them out - and also, all the territory the U.S. took on becomes a permanent part of the nation. This means Cuba and the Philippines especially, with Spain essentially becoming a U.S. territory as well. These would have to be territories, not states, and be semi-autonomous while still paying taxes to the U.S. Sure, it makes America a colossal hypocrite on the whole "taxation without representation" thing, but this can't work if Uncle Sam isn't at least somewhat of a dick.
The taxation pays for the occupation forces, while an improvement in quality of life prevents uprisings. Meanwhile, the U.S. government looks at WWI as a territory grab. It makes gains in the Middle East, all the while snapping up territory where it can - by the 20s it controls a number of waterways, including all of Panama and the Suez. It uses this influence, as well as support of the Mensheviks, to smother the Communist government in Russia before it gets too powerful, effectively destroying communism before the 30s. Yes, it still has its backers, even stateside, but it's seen as a fad, not as a force.
Without a strong government to replace the Tsars, Russia falls apart, and the U.S. puts in a puppet, worming its hands into Russia's government and mobilizing its military. At the time, Hitler isn't viewed as nearly as dangerous as he really was - Russia's job is to snuff out the Japanese, with whom America had long anticipated a war. By 1940, America cuts enough deals to make Russia a U.S. territory - essentially the Russian leadership sees it as, "Go with these guys or collapse." Especially when Hitler reveals himself to be their worst nightmare.
WWII goes much as it does IOTL without all the pesky communism - and in the wake of Europe's devastation, here comes America to the rescue with a plan to make the defeated nations of Germany and Italy into U.S. territories. Since America pays for it, the U.K. and France go along with it, absorbing a few other destroyed areas in the process until all of Europe except Britain, France, Switzerland, a chunk of Scandinavia, and maybe southeastern Europe are all part of the United States.
Much the same happens in Asia, putting China and Korea in American hands to rebuild - paid for with taxes from their many territories, of course. America then snaps up some other areas such as Indonesia and controls the entire Pacific. All that's left is Southern Asia, and a war between India and Pakistan leaves both nations ripe for "rebuilding."
Decolonization of Africa invites the Americans in as peacekeepers, and uprisings in Central America do the same. Eventually America is so overwhelmingly powerful and pervasive that it decides it should run the entire world and looks for any excuse it can to invade holdout nations. Most territories are pro-U.S. because of the improvement of quality of life from where they were when they joined (which doesn't take much,) so they gladly send mercenaries to help conquer the world. And through sheer force, the United States of America conquers, breaks down, and rebuilds in its own image every nation on the planet.
And at this point, it has a hell of a lot of rebellions to put down.