U.S. Presidents of the 20th century include Theodore Roosevelt and Harry Truman. As of 2015, George W. Bush and Barack Obama are the two main presidents of the 21st century.
Theodore Roosevelt became president following the assassination of President William McKinley right after the turn of the 20th century. At that time, Roosevelt was the youngest president in U.S. history. Among his many achievements were his promotion of conservation of natural lands and overseeing the building of the Panama Canal.
Harry Truman took over the presidency from Franklin D. Roosevelt while the United States was in the middle of fighting World War II. He is perhaps most famous, or infamous, for ordering the dropping of the atomic bomb on the Japanese cities of Hiroshima and Nagasaki. The attack caused a massive loss of civilian life and also brought the war with Japan to a speedy conclusion.
George W. Bush was elected president during the first year of the new millennium, and he formally took office in January of the following year. He was president during the terrorist attacks of Sept.11, 2001, and started a U.S. war on terrorism and a war on Iraq.
President Barack Obama made history in January of 2009 by becoming the nation's first African American president.