Some of the most important events in 20th-century America include the atomic bombings in Japan, the assassination of Archduke Franz Ferdinand of Austria that led to World War I and the fall of the Berlin Wall. The assassination took place on June 28, 1914, when Franz Ferdinand was out riding with his wife, and contributed to the beginning of World War I.
Another 20th-century significant event is the bombing of the Japanese cities, Hiroshima and Nagasaki. In World War II, the United States bombed both of these countries killing thousands of people in an attempt to bring the war to a close. Although Germany surrendered in 1944, the Japanese had refused to surrender and continued to fight. It was President Truman who decided that it would be better to use large force to hopefully end the war in 1945 instead of losing an expected one million American soldiers in a ground fight. Secretary of War Henry Stimson, several of the Manhattan Project scientists and General Eisenhower all opposed the idea.
The fall of the Berlin Wall occurred in 1989. The wall was up for 28 years and only came down because of the political changes happening in Europe. This freed the East Germans from their life of confinement.