The New American Frontier

Starting in 1890, the year the United States Census Bureau officially declare the last American frontier populated in the West, how large could the United States PLAUSIBLY become? What regions would the US of 2004 occupy? What differences would this have (on the United States and on the world)?

After the Spanish American War, the US annexes Cuba and the Phillipines as 'overseas territories' so that both become scheduled to being states one day. I think the Phillipines were taken with the idea of being set free eventually.

The US elects William Jennings Bryan in the late 1890s. A Populist president is more sympathetic to radical revolutionanies then just beginning to be heard in Mexico and withdraws a lot of official US support for the Diaz regime, while simultaenously attacking Big Business here at home so that they too can send less money to prop up Diaz.

This backfires, however, the Revolution comes too soon. Diaz is able to quell it, though far from completely, while seeking more aid from Europe. Germany becomes his big supporter. The US sends lots more private aid to the revolution than in OTL and some covert.

In WWI, Germany's Zimmermann Telegram conspiracy becomes an abortive invasion of the US, which draws the US into WWI much earlier. The US response to Mexico's invasion involves massive support of the Revolution. Diaz falls within months

After the war, US troops prevent the chaos which ensued in this century. Drawing on over 10 years of friendship developed in the time of covert support and not wanting to follow the example of the rest of Latin America pro-US forces in the Revolution become dominant, seeking annexation and eventualy statehood. The other nations of Central America, most of which, except Panama and Belize, came from Mexico anyway also eventually throw in their lot with America. Mexico become the 47th state in 1935. Guatemala becomes the 55th State in 1962

American Empire

WWI lasts twice as long, kills twice as many people, costs twice as much money, but the Entente does better and stays in business. Maybe they get the Dardanelles open to keep Russia in the war, maybe they keep plan Michel and Belgium is successfully defended, etc.
So the US begins buying up the Belgian, French, British empires, piece by piece. When they have it all, they force the Entente to make peace by cutting off their money and buy the German colonies from a bankrupt Reich.
This leaves them with an empire. They appoint a bunch of locals as a government and go home, a la the Phillipines. As part of the US empire everybody leaves them alone, but they are de facto independent.

Thats monsterous!