20th century

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The Earth seen from Apollo 17. The second half of the 20th century saw an increase of interest in space exploration.

The 20th century of the Christian Era began on January 1, 1901 and ended on December 31, 2000. According to the Gregorian calendar, 2000 was the first century leap year since 1600.

The Russian Empire, the German Empire, the Ottoman Empire, and the Austro-Hungarian Empire dissolved in the first half of the century, with all but the British Empire collapsing during the course of World War I. The inter-war years saw the Great Depression cause a massive disruption to the world economy. Shortly afterwards, World War II broke out, pitting the Allied powers (chiefly the Soviet Union, the United Kingdom, and France) against the Axis powers (chiefly Nazi Germany, and the Empire of Japan), which eventually resulted in a total victory for the Allies, but at the cost of over 60 million lives and complete devastation of countless nations. As means to prevent future world wars, the United Nations was formed, however disagreements between the two new superpowers, the Soviet Union and the United States, resulted in the Cold War, which dominated geopolitical life for 40 years. The Soviet Union broke up in 1991, resulting in the United States taking on sole superpower status, although by the end of the century China, India, and the European Union had greatly increased their influence.

The century saw a major shift in the way that vast numbers of people lived, as a result of political, ideological, economic, social, cultural, scientific, technological, and medical change and progress. Terms like ideology, world war, genocide, and nuclear war entered common usage. Scientific discoveries, such as the theory of relativity and quantum physics, drastically changed the worldview of scientists, causing them to realize that the universe was fantastically more complex than previously believed, and dashing the hopes at the end of the 19th century that the last few details of scientific knowledge were about to be filled in. Accelerating scientific understanding, more efficient communications, and faster transportation transformed the world in those hundred years more rapidly and widely than at any time in the past. It was a century that started with horses, simple automobiles, and freighters but ended with luxury sedans, cruise ships, airliners and the space shuttle. Horses, Western society's basic form of personal transportation for thousands of years, was replaced by automobiles and buses within the span of a few decades. These developments were made possible by the large-scale exploitation of fossil fuel resources (especially petroleum), which offered large amounts of energy in an easily portable form, but also caused widespread concerns about pollution and long-term impact on the environment. Humanity explored outer space for the first time, even taking their first footsteps on the Moon.

Mass media, telecommunications, and information technology (especially computers, paperback books, public education, and the Internet) put the world's knowledge more widely available to people. Many people's view of the world changed significantly as they became much more aware of the struggles of others and, as such, became increasingly concerned with human rights. Advancements in medical technology also improved the welfare of many people: the life expectancy of the world increased from 35 years to 65 years. Rapid technological advancements, however, also allowed warfare to reach unprecedented levels of destruction. World War II alone killed over 60 million people, while nuclear weapons gave humankind the means to destroy itself in a very short period of time. The world also became more culturally homogenized than ever with developments in transportation and communications technology, popular music and other influences of Western culture, international corporations, and what was arguably a true global economy by the end of the century.

Summary

The early arms races 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, resulting from the trench warfare created by these weapons and the strategy of the armies involved, 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 neighbors. 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.