The Soviet Union, also known as the Union of Soviet Socialist Republics (USSR), broke up in 1991. This event was a significant turning point in modern history and had far-reaching consequences for both the Soviet Union and the world.

1. Background:
The Soviet Union was formed in 1922 after the Bolshevik Revolution in 1917, which overthrew the Russian Provisional Government. Under the leadership of Vladimir Lenin, the Communist Party took control, and the country underwent a radical transformation into a socialist state.

2. Factors leading to the breakup:
Several factors contributed to the breakup of the Soviet Union:

- Economic issues: The Soviet economy was plagued by inefficiency, corruption, and a lack of innovation. Centralized planning and a command economy proved to be ineffective in addressing the needs of a modern industrialized society.

- Nationalism and ethnic tensions: The Soviet Union was a diverse federation of republics, each with its own distinct culture, language, and history. Over time, nationalist sentiments grew, leading to demands for greater autonomy or even independence.

- Political reforms: The policies of Soviet leader Mikhail Gorbachev, such as glasnost (openness) and perestroika (restructuring), aimed to address the economic and political issues facing the country. However, these reforms also unleashed long-suppressed political aspirations and grievances, further fueling the breakup.

3. The Baltic States:
The first cracks in the Soviet Union appeared in the Baltic states of Estonia, Latvia, and Lithuania. These countries had been forcibly incorporated into the Soviet Union in 1940 and had never fully accepted Soviet rule. In 1991, they declared their independence, which was recognized by most of the international community.

4. Coup attempt and dissolution:
In August 1991, a group of hardline Communist Party members attempted a coup against Gorbachev's government. The coup failed, but it accelerated the disintegration of the Soviet Union. Following the failed coup, the individual republics began to assert their independence, and the central government's authority rapidly eroded.

On December 8, 1991, the leaders of Russia, Ukraine, and Belarus signed the Belavezha Accords, which declared the Soviet Union dissolved and established the Commonwealth of Independent States (CIS) as a successor entity.

5. Legacy and aftermath:
The breakup of the Soviet Union had profound consequences. The newly independent states faced numerous challenges, including economic restructuring, political instability, and the need to establish new institutions. Russia emerged as the largest and most influential successor state, inheriting the Soviet Union's seat on the United Nations Security Council and its nuclear arsenal.

Internationally, the end of the Cold War ushered in a new era of global politics. The collapse of the Soviet Union marked the formal end of the bipolar world order dominated by the United States and the Soviet Union. It also led to a wave of democratic movements and the spread of capitalism in Eastern Europe and other parts of the world.

In conclusion, the Soviet Union broke up in 1991 due to a combination of economic, political, and nationalist factors. The breakup had significant repercussions both domestically and internationally, reshaping the global geopolitical landscape.