The Soviet leader whose doctrine prevented the Eastern European states from breaking away from the Soviet Union

The Soviet leader who implemented the doctrine that prevented Eastern European states from breaking away from the Soviet Union was Joseph Stalin. However, the specific doctrine you're referring to is known as the Brezhnev Doctrine, named after Leonid Brezhnev, who was the General Secretary of the Central Committee of the Communist Party of the Soviet Union.

Here are the steps to answer your question:

1. Identify the Soviet leader: The doctrine was named after Leonid Brezhnev, who served as the General Secretary of the Central Committee of the Communist Party of the Soviet Union from 1964 until his death in 1982.

2. Understand the doctrine: The Brezhnev Doctrine was a policy that was announced to justify the Soviet Union's intervention in the Czechoslovak Socialist Republic in 1968. This doctrine stated that the Soviet Union had the right to intervene in any socialist country where the socialist system was under threat.

3. Connect the doctrine to the Eastern European states: The Brezhnev Doctrine was used to maintain control over Eastern European states and prevent them from breaking away from the Soviet Union's influence. It was essentially a guarantee that the Soviet Union would intervene in the affairs of its satellite states if it deemed necessary to preserve communism in that country.

So, the Soviet leader whose doctrine prevented the Eastern European states from breaking away from the Soviet Union was Leonid Brezhnev.