Question

What accounts for why the Soviet Union decided to formalize its East European alliance through the Warsaw Pact?

Answer 1

West Germany was accepted into NATO and re-militarized, Which threatened the Soviet Union. So, the Soviet Union formalized its East European alliance with the Warsaw Pact. This created a counter-balance to the NATO alliance

Answer 2

To create a counter-balance to the NATO alliance. The Warsaw Pact included all nations in the Soviet Bloc, meanwhile NATO having the rest of the western nations. The Soviet Union wanted to make something equivalent to that.