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

The twentieth century of the Common Era began on January 1, 1901 and ended on December 31, 2000, according to the Gregorian calendar. The century saw a remarkable shift in the way that vast numbers of people lived, as a result of technological, medical, social, ideological, and political innovation. Terms like ideology, world war, genocide, and nuclear war entered common usage.

The period witnessed radical changes in many areas of human endeavors. Scientific discoveries, such as the theory of relativity and quantum physics, drastically changed the world-view 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 steam-powered ships and ended with the space shuttle. Horses and other pack animals, Western society's basic form of personal transportation for thousands of years, were replaced by automobiles 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 great amounts of energy in an easily portable and storable form, but also caused widespread concerns about pollution and our 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 the Internet) put the world's knowledge at the disposal of nearly everyone in most industrialized societies and many people in developing countries as well. Many people's view of the world changed significantly as they became much more aware of the suffering and struggles of others and, as such, became increasingly concerned with human rights. In the latter half of the century especially, mankind became aware of the vast scale on which it had affected the planet, and took steps to minimize its damage of the planet's fragile ecosystems. Advancements in medical technology also improved the welfare of many people on the planet; life expectancy increased dramatically from the mid-30s to the mid-60s worldwide during the century. The healthiest countries had life expectancies of over 80 years by the turn of the millennium. Rapid technological advancements, however, also allowed warfare to reach an unprecedented low; World War II alone killed over 60 million people, while nuclear weaponry 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 massive arms race of the 20th century culminated in 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 (although some portion of the British Empire remained until the 1997 handover of Hong Kong to China) 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. However, its economy was ruined by World War I, and its empire began to shrink, producing a growing power vacuum in Europe. 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 off by Nazi Germany's aggressive expansion at the expense of its neighbours. Meanwhile, Japan had rapidly industrialized and 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 States, the United Kingdom, 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 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, the most devastating ones yet to have been developed, 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 helped the United States 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. 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.

After 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.

Perhaps the first major policy of the United Nations was the creation of Israel, a country created as a homeland for the Jewish population. This infuriated some Muslims who had inhabited the area for over a thousand years. Some surrounding Arab countries quickly declared war on Israel, sparking a conflict which became one of the most volatile global conflicts of the latter half of the century. Behind nearly unilateral support from the United States and other Western nations, Israel waged several wars with its Arab neighbors in 1948, 1958, 1967, 1973, and 1982. Anger over the Israeli capture of Palestinian lands during the Six-Day War of 1967 led to a wave of attacks by the Palestine Liberation Organization (PLO), during the 1970s. Negotiations during the late 1980s and 1990s led to the PLO transitioning officially to a political organization; however, other Palestinian groups continued attacks against Israel. A derailing of the peace process during the late 1990s led to a resumption of attacks around the turn of the millennium. The political situation in the Middle East was further complicated as questions arose over the influence of Islam on the governments of many Middle Eastern countries; perceived human rights violations by many of these countries; the possession of vast petroleum and natural gas reserves in the Persian Gulf region; and the Soviet war in Afghanistan that fueled such rogue Islamic groups as Al-Qaeda that would change the political landscape in the following century.