The USSR's formation of the Warsaw Pact was a reaction to what?

The Soviet response to the arrangement of the North Atlantic Treaty Organization was a show-work out; they made the 'defensive' Warsaw Pact coalition of their own from countries they officially controlled.

Explanation:

The Soviet Union had forced its will in a large portion of the zones of Central Europe it involved, and controlled military and remote territories such as Poland, Czechoslovakia, Hungary, Romania and Bulgaria. They didn't require an adversary collusion until 1955, after West Germany was reestablished to its very own administration and joined NATO. Subsequently the USSR experienced the presence of making a 'defensive' partnership and made the Warsaw Pact.

The Warsaw Pact was designed as a counter balance to NATO. It was essentially The Soviet Union's version of NATO.