Militarism is defined as:

the belief or desire of a government or people that a country should maintain a strong military capability and be prepared to use it aggressively to defend or promote national interests.

Alternative definitions include "aggressiveness that involves the threat of using military force", the "glorification of the ideals of a professional military class" and the "predominance of the armed forces in the administration or policy of the state" (see also: stratocracy and military junta).

Militarism has been a significant element of the imperialist or expansionist ideologies of several nations throughout history. Prominent examples include the Ancient Assyrian Empire, the Greek city state of Sparta, the Roman Empire, the Aztec nation, the Kingdom of Prussia, the British Empire, the Empire of Japan, the Russian Soviet Federative Socialist Republic (which would later become part of the Soviet Union), the Italian Colonial Empire during the reign of Benito Mussolini, Nazi Germany and American Imperialism.

World War I (WWI), which was predominantly called the World War or the Great War from its occurrence until 1939 (World War II), and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. It involved all the world's great powers, which were assembled in two opposing alliances: the Allies (based on the Triple Entente of the United Kingdom, France and Russia) and the Central Powers (originally centred around the Triple Alliance of Germany, Austria-Hungary and Italy; but, as Austria–Hungary had taken the offensive against the agreement, Italy did not enter into the war). These alliances both reorganised (Italy fought for the Allies), and expanded as more nations entered the war. Ultimately more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. More than 9 million combatants were killed, largely because of enormous increases in lethality of weapons, thanks to new technology, without corresponding improvements in protection or mobility. It was the sixth-deadliest conflict in world history, subsequently paving the way for various political changes such as revolutions in the nations involved.