The United States' boundaries have expanded over the centuries—and at the same time, Americans' ideas about their country have grown as well. The nation the world knows today was shaped by centuries of thinkers and events. Today, the United States of America is the lone super power in the world. The United States is very strong. Power, however, is not the only thing that comes with being a world leader. As a world leader, America also has a lot of responsibility to the rest of the world. In the twentieth century, the United States struggled to balance its power with its responsibility in new ways.