Most modern forms of communism are grounded at least nominally in Marxism, a theory and method conceived by Karl Marx during the 19th century. Marxism subsequently gained a widespread following across much of Europe and throughout the late 1800s its militant supporters were instrumental in a number of failed revolutions on that continent. During the same era, there was also a proliferation of communist parties which rejected armed revolution, but embraced the Marxist ideal of collective property and a classless society. Although Marxist theory suggested that the places ripest for social revolution—either through peaceful transition or by force of arms—were industrial societies, communism was mostly successful in underdeveloped countries with endemic poverty such as the Russian Empire and the Republic of China. In 1917, the Bolshevik Party seized power during the Russian Revolution and created the Soviet Union, the world's first self-declared socialist state. The Bolsheviks thoroughly embraced the concept of proletarian internationalism and world revolution, seeing their struggle as an international rather than a purely regional cause. This was to have a phenomenal impact on the spread of communism during the 20th century as the Soviet Union installed new Marxist–Leninist governments in Central and Eastern Europe following World War II and indirectly backed the ascension of others in the Americas, Asia and Africa. Pivotal to this policy was the Communist International, also known as the Commintern, formed with the perspective of aiding and assisting communist parties around the world and fostering revolution. This was one major cause of tensions during the Cold War as the United States and its military allies equated the global spread of communism with Soviet expansionism by proxy.
By 1985, one-third of the world's population lived under a Marxist–Leninist system of government in one form or another. However, there was significant debate among communist and Marxist ideologues as to whether most of these countries could be meaningfully considered Marxist at all since many of the basic components of the Marxist system were altered and revised by such countries. The failure of these governments to live up to the ideal of a communist society as well as their general trend towards increasing authoritarianism has been linked to the decline of communism in the late 20th century. With the dissolution of the Soviet Union in 1991, several Marxist–Leninist states repudiated or abolished the ideology altogether. By the 21st century, only a small number of Marxist–Leninist states remained, namely Cuba, Vietnam and Laos. Despite retaining a nominal commitment to communism, China has essentially ceased to be governed by the principles of Maoism, reverting to an authoritarian regime with a mixed economy.