Summary

The early arms race of the 20th century escalated into a war which involved many powerful nations: World War I (1914–1918). This war drastically changed the way war was fought, as new inventions such as machine guns, tanks, chemical weapons, and grenades created stalemates on the battlefield and millions of troops were killed with little progress made on either side. After more than four years of trench warfare in western Europe, and 20 million dead, those powers who had formed the Triple Entente (France, Britain, and Russia, later replaced by the United States and joined by Italy) emerged victorious over the Triple Alliance (Germany, Austria-Hungary, and the Ottoman Empire). In addition to annexing much of the colonial possessions of the vanquished states, the Triple Entente exacted punitive restitution payments from their former foes, plunging Germany in particular into economic depression. The Russian Empire was plunged into revolution during the conflict and transitioned into the first ever communist state, and the Austro-Hungarian and Ottoman empires were dismantled at the war's conclusion. World War I brought about the end of the royal and imperial ages of Europe and established the United States as a major world military power.

At the beginning of the period, Britain was arguably the world's most powerful nation. Fascism, a movement which grew out of post-war angst and accelerated by the Great Depression of the 1930s, gained momentum in Italy, Germany and Spain in the 1920s and 1930s, culminating in World War II (1939–1945), sparked by Nazi Germany's aggressive expansion at the expense of its neighbours. Meanwhile, Japan had rapidly transformed itself into a technologically advanced industrial power. Its military expansion into eastern Asia and the Pacific Ocean helped to bring the United States into World War II. Germany was defeated by the Soviet Union in the east and by the D-Day invasion of the United Kingdom, the United States, Canada, and Free France from the west. The war ended with the dropping of two atomic bombs on Japan. Japan later became a U.S. ally with a powerful economy based on consumer goods and trade. Germany was divided between the western powers and the Soviet Union; all areas recaptured by the Soviet Union (East Germany and eastward) were essentially transitioned into Soviet puppet states under communist rule. Meanwhile, western Europe was influenced by the American Marshall Plan and made a quick economic recovery, becoming major allies of the United States under capitalist economies and relatively democratic governments.

World War II left about 60 million people dead. When the conflict ended in 1945, the United States and the Soviet Union emerged as very powerful nations. Allies during the war, they soon became hostile to one other as the competing ideologies of communism and democratic capitalism occupied Europe, divided by the Iron Curtain and the Berlin Wall. The military alliances headed by these nations (NATO in North America and western Europe; the Warsaw Pact in eastern Europe) were prepared to wage total war with each other throughout the Cold War (1947–91). The period was marked by a new arms race, and nuclear weapons were produced in the tens of thousands, sufficient to end most life on the planet had they ever been used. This is believed by some historians to have staved off an inevitable war between the two, as neither could win if their full nuclear arsenals were unleashed upon each other. This was known as mutually assured destruction (MAD). Although the Soviet Union and the United States never directly entered conflict with each other, several proxy wars, such as the Korean War (1950–1953) and the Vietnam War (1957–1975), were waged to contain the spread of communism.

After World War II, most of the European-colonized world in Africa and Asia gained independence in a process of decolonization. This, and the drain of the two world wars, caused Europe to lose much of its long-held power. Meanwhile, the wars empowered several nations, including the UK, U.S., Russia, China and Japan, to exert a strong influence over many world affairs. American culture spread around the world with the advent of Hollywood, Broadway, rock and roll, pop music, fast food, big-box stores, and the hip-hop lifestyle. British culture continued to influence world culture, including the "British Invasion" into American music, leading many top rock bands (such as Swedish ABBA) to sing in English. The western world and parts of Asia enjoyed a post-World War II economic boom. After the Soviet Union collapsed under internal pressure in 1991, a ripple effect led to the dismantling of communist states across eastern Europe and their rocky transitions into market economies.

Following World War II, the United Nations was established as an international forum in which the world's nations could get together and discuss issues diplomatically. It has enacted laws on conducting warfare, environmental protection, international sovereignty, and human rights, among other things. Peacekeeping forces consisting of troops provided by various countries, in concert with various United Nations and other aid agencies, has helped to relieve famine, disease, and poverty, and to contain local wars and conflicts. Europe slowly united, politically and economically, into what eventually became the European Union, which consisted of 15 European countries by the end of the century.

In approximately the last third of the century, concern about humankind's impact on the Earth's environment caused environmentalism to become a major citizen movement. In many countries, especially in Europe, the movement was channeled into politics partly through Green parties, though awareness of the problem permeated societies. By the end of the century, some progress had been made in cleaning up the environment though pollution continued apace.^{[citation needed]} Increasing awareness and pessimism over global warming began in the 1980s, sparking one of the most heated social and political debates by the turn of the century.

Medical science and the Green Revolution in agriculture enabled the world's population to grow from about 1.6 billion to about 6.0 billion. This rapid population increase quickly became a major concern and directly caused or contributed to several global issues, including conflict, poverty, major environmental issues, and severe overcrowding in some areas.

The nature of change

Due to continuing industrialization and expanding trade, many significant changes of the 20th century were, directly or indirectly, economic and technological in nature. Inventions such as the light bulb, the automobile, and the telephone in the late 1800s, followed by supertankers, airliners, motorways, radio, television, antibiotics, frozen food, computers and microcomputers, the Internet, and mobile telephones affected the quality of life for great numbers. Scientific research and technological development was the force behind vast changes in everyday life, to a degree which was unprecedented in human history.