Final answer:
The communist version of socialism peaked in the 1960s, with the Soviet Union as a major power, but collapsed by the 1990s due to economic inefficiencies and political unrest.
Step-by-step explanation:
The communist version of socialism reached its peak in the 1960s and by the 1990s it was in a state of collapse.
During the 1960s, the Soviet Union was at the height of its power with approximately one-third of the world's population living under communist regimes. Notably, the USSR and People's Republic of China were major communist powers. By the end of the 20th century, however, nationalism had eroded the unity of the working class Marx envisioned, and the appeal of communism diminished significantly. Communist countries struggled economically and politically, failing to meet the egalitarian visions of Karl Marx. This was evidenced by inefficient economic systems, such as the collectivization of agriculture in the USSR, which contributed to famine and millions of deaths.
The eventual collapse of communist states like the Soviet Union came about due to internal economic difficulties, political unrest, and the inability to keep pace with technological and economic developments seen in capitalist countries. By the 1990s, few nations remained under communist rule, marking the decline of this era of socialism.