The 20th century (1901-2000) has been called "The American Century" and "The Bloodiest Century", because during most of it America was fighting wars for democracy. The 20th century was the century of World War I, World War II, Korea, Vietnam, the Iran-Iraq War, and Iraq, as well as smaller conflicts such as Rwanda and the Falkland Islands.
However, the 20th century was also a century of inventions. The electronic computer was invented in the 20th century, and popularized by pioneers like Bill Gates of Microsoft and Steve Jobs and Steve Wozniak of Apple Computers. Many inventions that we take for granted today, such as antibiotics, plastic, SCUBA gear, the transistor, the microwave oven, radar, polio vaccine, the airplane, and the cellular phone appeared in the 20th Century.  On the other hand, some inventions, like the telegraph and the steamship, became obsolete and all but disappeared.
In the United States of America
A major theme of the twentieth century was the decline of the worldwide empires of the previous century. Australia became independent on January 1, 1901. The end of World War 1 led to independence for several European countries such as Poland and Hungary. The following two decades saw independence for Egypt, Saudi Arabia and Iraq, among others, but the Second World war weakened the colonial powers so much that they granted independence to most of their colonies. India, Indonesia and Israel were among ten countries gaining independence in the 1940s, became independent first, followed by a dozen African states in 1960, and nearly every remaining African and Caribbean state in the two following decades. By 1980 Hong Kong (British) was the only overseas possession with more than one million inhabitants. Finally the 1990s saw the breakup of the Soviet Union and Yugoslavia - eighteen new countries were founded in a few years after forty years of European stability.
In contrast to this wave of new nations, a few countries lost their freedom. Tibet, Hyderabad and Montenegro fell to larger nations by war or politics; East Germany and South Yemen joined their larger neighbors voluntarily and peacefully.
In all, over a hundred countries had became independent by 2000.