Reason: "Manifest Destiny, a phrase coined in 1845, is the idea that the United States is destined—by God, its advocates believed—to expand its dominion and spread democracy and capitalism across the entire North American continent. The philosophy drove 19th-century U.S. territorial expansion and was used to justify the forced removal of Native Americans and other groups from their homes. The rapid expansion of the United States intensified the issue of slavery as new states were added to the Union, leading to the outbreak of the Civil War"
As the war in Europe erupted in 1914, the President of the United States, Woodrow Wilson, proclaimed an official policy of neutrality in regard to the war. That policy was hard to maintain, however, because the warring parties in Europe were battling over the seas and this affected shipping coming from the United States. Britain had blockaded Germany, and the Germans had responded by launching submarine (U-boat) warfare against vessels coming to deliver supplies to Britain. When the passenger liner <em>Lusitania </em>was sunk by the Germans in 1915, killing over 1,000 people including 120 Americans, the USA was almost drawn into the war at that point.