The century had the first global-scale
total wars between world powers across continents and oceans in
World War I and
World War II.
Nationalism became a major political issue in the world in the 20th century, acknowledged in
international law along with the right of nations to
self-determination, official decolonization in the mid-century, and nationalist-influenced armed regional-conflicts.
The century saw a major shift in the way that many people lived, with changes in politics, ideology, economics, society, culture, science, technology, and medicine. The 20th century may have seen more technological and scientific progress than all the other centuries combined since the dawn of civilization. Terms like ideology,
nuclear war entered common usage. 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 was more complex than previously believed, and dashing the hopes (or fears) at the end of the 19th century that the last few details of scientific knowledge were about to be filled in. It was a century that started with horses, simple automobiles, and
freighters but ended with
cruise ships, global
commercial air travel and the
space shuttle. Horses, Western society's basic form of personal transportation for thousands of years, were replaced by automobiles and buses within a few decades. 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 explored
space for the first time, taking their first footsteps on the
World powers and empires in 1914, just before the First World War.
Mass media, telecommunications, and information technology (especially computers,
public education, and the
Internet) made the world's knowledge more widely available. Advancements in
medical technology also improved the health of many people: the global life expectancy 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 annihilate itself in a short time. However, these same wars resulted in the destruction of the Imperial system. For the first time in human history, empires and their wars of expansion and colonization ceased to be a factor in international affairs, resulting in a far more globalized and cooperative world. The last time major powers clashed openly was in 1945, and since then, violence has seen an unprecedented decline.
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 20th century.
Technological advancements during
World War I changed the way war was fought, as new inventions such as
chemical weapons, and
aircraft modified tactics and strategy. After more than four years of
trench warfare in western Europe, and 20 million dead, the powers that had formed the
Triple Entente (
Russia, later replaced by the
United States and joined by
Romania) emerged victorious over the
Central Powers (
Ottoman Empire and
Bulgaria). In addition to annexing much 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 regime of Tsar
Nicholas II was overthrown during the conflict, Russia became the first communist state, and the Austro-Hungarian and Ottoman empires were dismantled at the war's conclusion.
At the beginning of the period,
Britain was the world's most powerful nation,
acted as the world's policeman for the past century. 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. Its military expansion into eastern Asia and the Pacific Ocean culminated in
a surprise attack on the United States,
bringing it into World War II. After some years of dramatic military success, Germany was
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
Free France from the west. The war ended with the
dropping of two atomic bombs on Japan. Japan later became a western ally with an economy based on the manufacture of
consumer goods and trade. Germany
was divided between the Western powers (
West Germany) and the
Soviet Union; all areas recaptured by the Soviet Union (
East Germany and eastward) became Soviet
puppet states under communist rule. Meanwhile, Western European countries were 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 the major world powers. Allies during the war, they soon became hostile to one another 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) threatened each other with total war in what was called 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 human life on the planet had a large-scale nuclear exchange ever occurred. The size of the nuclear arsenals is believed by many historians to have staved off war between the two, as the consequences were too great to bear. The policy of massive nuclear attack, knowing a similar counterattack would be forthcoming, was called
mutually assured destruction (MAD). However, several
proxy wars, such as the
Korean War (1950–1953) and the
Vietnam War (1957–1975), were waged as the United States implemented its worldwide "
containment" policy against communism.
After World War II, most of the
European-colonized world in Africa and Asia gained independence in a process of
decolonization. Meanwhile, the wars empowered several nations, including the UK, USA, Russia, China and Japan, to exert a strong influence over many world affairs.
American culture spread around the world with the advent of the
Hollywood motion picture industry,
rock and roll,
big-box stores, and the
hip-hop lifestyle. Britain continued to influence world
culture, including the
British Invasion (
The Rolling Stones,
The Beatles) into
American music, leading many
rock bands from other countries (such as Swedish
ABBA) to sing in English. The western world and parts of Asia enjoyed a
post–World War II economic expansion. After the Soviet Union
collapsed under internal pressure in 1991, the communist governments of the
Eastern bloc were dismantled, followed by awkward transitions into market economies.
Following World War II, the United Nations, successor to the League of Nations, was 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
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.
In the last third of the century, concern about humankind's impact on the Earth's
environment made environmentalism popular. In many countries, especially in Europe, the movement was channeled into politics through
Green parties. Increasing awareness of
global warming began in the 1980s, commencing decades of social and political debate.
is a major technological advancement in this century.
The nature of innovation and change
Due to continuing industrialization and expanding trade, many significant changes of the century were, directly or indirectly, economic and technological in nature. Inventions such as the
light bulb, the automobile, and the telephone in the late 19th century, followed by
airliners, motorways, radio, television,
frozen food, computers and
microcomputers, the Internet, and
mobile telephones affected people's
quality of life across the developed world. Scientific research, engineering professionalization and technological development drove changes in everyday life.
At the beginning of the century, strong discrimination based on race and sex was significant in general society. Although the
Atlantic slave trade had ended in the 19th century, the fight for equality for non-white people in white-dominated societies of North America, Europe, and South Africa continued. During the century, the social taboo of
sexism fell. By the end of the 20th century, women had the same legal rights as men in many parts of the world, and racism had come to be seen as abhorrent.
 Attitudes towards
homosexuality also began to change in the later part of the century.