1. In January, 1917, Germany had resumed its policy of unrestricted submarine warfare. Germany had halted its attacks on non-military vessels (which it suspected of carrying military supplies) after the furor over the sinking of the Lusitania in 1915. But now Germany was resuming attacks by its U-boats.
2. In February, 1917, the "Zimmerman Telegram" was intercepted by British intelligence and shared with the US. Germany's foreign minister, Arthur Zimmerman, had telegraphed an offer to Mexico's ambassador seeking Mexico's support in war vs. the United States in exchange for getting land back from the US.
3. On April 2, 1917, President Wilson made a powerful speech to Congress in which he argued that the nation needed to enter the war "to make the world safe for democracy." Wilson's speech was powerfully convincing, and four days later, Congress declared war.
Depends which time period. But it's probably the 20th century. The Cold War and WW2 plus others made the US impose democracy and capitalism for other countries.
