Isolationism is the idea that a country should not involve itself in other countries' affairs or develop entangling alliances. Many countries have practiced isolationism and many politicians have argued for the merits of isolationism throughout history, including in the United States.
In his famous "Farewell Address," George Washington warned the United States to "steer clear of permanent alliances with any portion of the foreign world," meaning to avoid getting tangled up in alliances with European countries which might eventually draw the young nation into war, but Washington was not a die-hard isolationist; he still advocated that the U.S. maintain open communications with other countries to advance trade.
Isolationist feelings rose again in the United States during World War I; many American leaders worried about the United States showing favoritism to either side of the conflict, fearing that possible trade boycotts would have lasting damage on American businesses. Many American citizens fiercely opposed the idea of their country entering into a European conflict that they felt had little or nothing to do with them. Woodrow Wilson even won his reelection on the slogan "he kept us out of war." Ultimately, the sinking of the Lusitania drew the U.S. into World War I despite Wilson's efforts, ending America's isolationist policy.
Isolationism refers to the idea that it is in the best interest of a country to isolate themselves from world events and to not actively interfere in the affairs of another state, so as to prevent themselves from being drawn into potential conflicts. It has been most notably practiced by the American government during World War I and World War II, where the nation chose to maintain a position of neutrality in the midst of the outbreak of war on the European continent. It was only after repeated badgering and a change in wartime circumstances that eventually drove the Americans to diverge from their policy of isolationism to join the side of the Allies during both wars as late participants in the conflict.