-1900: THE UNITED STATES GROWS UP
With the close of the Civil War in 1865, the U.S. was minus the free help of 4 million slaves but plus a determination to expand all across the continent and beyond. For the rest of the 1800s, mostly Republican presidents rode out economic downturns and political scandals while the U.S. economy steadily caught up with the world's only international superpower: Great Britain.
The U.S. got some technological breaks by inventing most of the useful gadgets in the late 1800s: the electric light, telephone, mechanical harvester, and mass production to name a few. Not held back by the need to support a large military or defend an empire, the U.S. poured all its capital into growth.
Because slavery was no longer an issue, new states got created as soon as they had the population to support a government. San Francisco was well established as the Queen of the Pacific, supported by both California gold and Nevada silver. Railroads spanned the continent, the longest creation of mankind since the Great Wall of China. Ironically, much of the western railroad starting in California was built by imported Chinese labor.
By the time the 1900s neared, the U.S. was starting to cast a hungry eye overseas for more territory. The country had bought Alaska from the Russians (1867), stolen the Southwest from Mexico (1848), and settled out the Northwest with the British (1846). The Hawaiian Islands fell like an unguarded flower into Yankee hands. In 1898, the U.S. fought the weak colonial power of Spain to take away Cuba and the far-off Philippines. President Teddy Roosevelt sent the fleet parading around the world. The United States had the glory; coming up next would be the burden of being a world power.
-1945: U.S. AS WORLD POWER
The U.S. entered the 1900s with an empire, sort of, and enough military power to scare away other nations from attacking the New World. Most of the United States wanted to mind its own business, but other nations were building up fleets and armies and trolling for more empire.
When World War I broke out in Europe in 1914, the United States stayed neutral. That was hard to do, because the British had been mother country to the U.S., and the French had helped the U.S. free itself. They were fighting together against Germany and clearly needed help. Even worse, ships carrying Americans kept getting sunk by German submarines, and the Germans even hatched a crazy plot to take over Mexico and the American Southwest. After three years of neutrality, the U.S. finally pitched in the war on the side of the Allies.
It didn't really take much fighting, but the U.S. tipped the balance, and the Allies defeated Germany in what turned out to be Round One of a two-round world war; Round Two (World War II) came later.
U.S. President Wilson had great plans for making a fair peace guaranteed by an international League of Nations that could keep future wars from developing. Congressional Republicans wouldn't go along with letting the United States help guard the peace. So, after what amounted to a 21-year truce to make more weapons, the major nations (including the United States) plunged into an even more destructive World War II.
This time the reluctant dragon U.S. sat behind its oceans for two years before being awakened by a punch in the nose from Japan. Although the U.S. lost only 2 percent of the people that Russia did fighting World War II, it was enough to convince the country to stay active in world affairs and try to preserve the peace in the future.