Throughout its history the United States had been an expansionist nation. By the middle of the 19th century the United States had acquired all the territory in the continental United States from the Atlantic to the Pacific Oceans. In the last decade of the 19th century the United States joined European powers in imperialistic expansion. The new Manifest Destiny led to acquisition of overseas possessions in the Caribbean and the Pacific. This imperialistic drive laid the groundwork for the United States’ role as a World Power.

As a world power, the United States reluctantly entered World War I. Although victorious, disillusionment with the war and its results led Americans to retreat inwardly. Their sentiments helped to usher in an era of isolationism, and escapism. Post World War I was a period marked by a conflict between more simple rural values of the 19th century and the more complex urban values of the 20th century. As the standard of living rose, more individuals sought the “American Dream” The economic boom disappeared with the onset of the Great Depression – a depression that continued for a full decade not only in the United States but also throughout the world. To recover from the Depression Americans elected Franklin Roosevelt and turned to the federal government for help. President. Roosevelt and his New Deal sought to address the nation’s problems and in so doing permanently altered the role of the government in the American economy and in the lives of the American people.

The Roosevelt Administration abandoned isolationism in response to the rise of totalitarianism and entered the world stage. The attack on Pearl Harbor pushed the United States into World War II. Millions of brave men and women served bravely in the armed forces. The United States success during World War II required a total commitment of its citizens and resources. Mobilization led to extensive government involvement in the economy and greatly increased the power of the President. In August 1945, atomic bombs dropped on Japan ended the war. The story of the bomb’s creation and the decision of the United States to use it deserve close attention. These events changed the world. Much of the industrial world had experienced the destruction of battle, and unprecedented death. There were nearly 50 million casualties from the war and millions more homeless. Only the United States emerged with its homeland intact and its economy prosperous.

Diplomatic relationship among Allied Powers began to disintegrate as war ended. The war permanently ended traditional American isolationism.