The document summarizes key details about several wars involving the United States: World War I was caused by rising nationalism in Europe and alliances between countries that led to trench warfare. The US entered after German U-boats sank American ships. World War II saw the US enter to defeat enemy countries like Germany and Japan. It boosted the economy by creating jobs and demand for goods to support the troops. The Vietnam War was fought between North and South Vietnam and divided Americans. Returning soldiers faced protests at home instead of praise. The document also discusses how works like The Great Gatsby capture the psychological difficulties faced by soldiers returning from war like Vietnam.