20th century
The 20th century began on 1 January 1901, and ended on 31 December 2000. It was the 10th and last century in the 2nd millennium and was marked by new models of scientific understanding, unprecedented scopes of warfare, new modes of communication that would operate at nearly instant speeds, and new forms of art and entertainment. Population growth was also unprecedented, as the century started with around 1.6 billion people, and ended with around 6.2 billion.
The 20th century was dominated by significant geopolitical events that reshaped the political and social structure of the globe: World War I, the Spanish flu pandemic, World War II and the Cold War. Unprecedented advances in science and technology defined the century, including the advent of nuclear weapons and nuclear power, space exploration, the shift from analog to digital computing and the continuing advancement of transportation, including powered flight and the car. The Earth's sixth mass extinction event, the Holocene extinction, continued, and human conservation efforts increased.
Major themes of the century included decolonization, nationalism, globalization and new forms of intergovernmental organizations. Democracy spread, and women were given the right to vote in many countries in the world. Cultural homogenization began through developments in emerging transportation and information and communications technology, with popular music and other influences of Western culture, international corporations, and what is arguably a truly global economy by the end of the 20th century. Poverty was reduced and the century saw rising standards of living, world population growth, awareness of environmental degradation and ecological extinction. Cars, airplanes, and home appliances became common, and video and audio recording saw mass adoption. These developments were made possible by the exploitation of fossil fuel resources, which offered energy in an easily portable form, but also caused concern about pollution and long-term impact on the environment. Humans started to explore space, taking their first footsteps on the Moon. Great advances in electricity generation and telecommunications allowed for near-instantaneous worldwide communication, ultimately leading to the Internet. Meanwhile, advances in medical technology resulted in the near-eradication and eradication of many infectious diseases, as well as opening the avenue of biological genetic engineering. Scientific discoveries, such as the theory of relativity and quantum physics, profoundly changed the foundational models of physical science, forcing scientists to realize that the universe is 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.
Summary
At the beginning of the period, the British Empire was the world's most powerful nation, having acted as the world's policeman for the past century.Technological advancements during World War I changed the way war is now fought, as new inventions such as tanks, chemical weapons, and aircraft modified tactics and strategy. After more than four years of trench warfare in Western Europe, and up to 17 million dead, the powers that had formed the Triple Entente emerged victorious over the Central Powers. In addition to annexing many of the colonial possessions of the vanquished states, the Triple Entente exacted punitive restitution payments from them, plunging Germany in particular into economic depression. The Austro-Hungarian and Ottoman empires were dismantled at the war's conclusion. The Russian Revolution resulted in the overthrow of the Tsarist regime of Nicholas II and the onset of the Russian Civil War. The victorious Bolsheviks then established the Soviet Union, the world's first communist state.
File:GonsalvesPapuas1958.jpg|thumb|Dutch colonial officer with Papuans in the Baliem Valley, Dutch New Guinea, 1958
Fascism, a movement which grew out of post-war angst and which accelerated during the Great Depression of the 1930s, gained momentum in Italy, Germany, and Spain in the 1920s and 1930s, culminating in World War II, 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 and, along with Germany and Italy, formed the Axis powers. Japan's military expansionism in East Asia and the Pacific Ocean brought it into conflict with the United States, culminating in a surprise attack which drew the US into World War II.
After some years of dramatic military success, Germany is defeated in 1945, having been invaded by the Soviet Union and Poland from the East and by the United States, the United Kingdom, Canada, and France from the West. After the victory of the Allies in Europe, the war in Asia ended with the Soviet invasion of Manchuria and the dropping of two atomic bombs on Japan by the US, the first nation to develop nuclear weapons and the only one to use them in warfare. In total, World War II left some 60 million people dead.
File:Atomic cloud over Hiroshima - NARA 542192 - Edit.jpg|thumb|The mushroom cloud of the detonation of Little Boy, the first nuclear attack in history, on 6 August 1945 over Hiroshima, igniting the nuclear age with the international security dominating thread of mutual assured destruction in the latter half of the 20th century.
Following World War II, the United Nations, successor to the League of Nations, is established as an international forum in which the world's nations could discuss issues diplomatically. It enacted resolutions on such topics as the conduct of warfare, environmental protection, international sovereignty, and human rights. Peacekeeping forces consisting of troops provided by various countries, with various United Nations and other aid agencies, helped to relieve famine, disease, and poverty, and to suppress some local armed conflicts. Europe slowly united, economically and, in some ways, politically, to form the European Union, which consisted of 15 European countries by the end of the 20th century.
After the war, Germany is occupied and divided between the Western powers and the Soviet Union. East Germany and the rest of Eastern Europe became Soviet puppet states under communist rule. Western Europe is rebuilt with the aid of the American Marshall Plan, resulting in a major post-war economic boom, and many of the affected nations became close allies of the United States.
With the Axis defeated and Britain and France rebuilding, the United States and the Soviet Union were left standing as the world's only superpowers. Allies during the war, they soon became hostile to one another as their competing ideologies of communism and democratic capitalism proliferated in Europe, which became divided by the Iron Curtain and the Berlin Wall. They formed competing military alliances which engaged in a decades-long standoff known as the Cold War. The period is marked by a new arms race as the USSR became the second nation to develop nuclear weapons, which were produced by both sides in sufficient numbers to end most human life on the planet had a large-scale nuclear exchange ever occurred. Mutually assured destruction is credited by many historians as having prevented such an exchange, each side being unable to strike first at the other without ensuring an equally devastating retaliatory strike. Unable to engage one another directly, the conflict played out in a series of proxy wars around the world—particularly in China, Korea, Cuba, Vietnam, and Afghanistan—as the USSR sought to export communism while the US attempted to contain it. The technological competition between the two sides led to substantial investment in research and development which produced innovations that reached far beyond the battlefield, such as space exploration and the Internet.
File:Africa independence dates.PNG|thumb|The international community grew in the second half of the century significantly due to a new wave of decolonization, particularly in Africa. Most of the newly independent states, were grouped together with many other so called developing countries. Developing countries gained attention, particularly due to rapid population growth, leading to a record world population of nearly 7 billion people by the end of the century.
In the latter half of the century, most of the European-colonized world in Africa and Asia gained independence in a process of decolonization. Meanwhile, globalization opened the door for several nations to exert a strong influence over many world affairs. The US's global military presence spread American culture around the world with the advent of the Hollywood motion picture industry and Broadway, jazz, rock music, and pop music, fast food and hippy counterculture, hip-hop, house music, and disco, as well as street style, all of which came to be identified with the concepts of popular culture and youth culture. After the Soviet Union collapsed under internal pressure in 1991, most of the communist governments it had supported around the world were dismantled—with the notable exceptions of China, North Korea, Cuba, Vietnam, and Laos—followed by difficult transitions into market economies.