Who was the primary aggressor in Europe?

Germany

Explanation:

There were the primary aggressors of their very hostile relationship with other countries. Germany invaded Poland which wasn't supported by France and Britain and as a result of this the Germans also invaded these countries.

They also broke the non aggression between them and the Soviet Union by invading the country also and several others such as Greece, Yugoslavia etc.

They were the primary leaders and cause of the Holocaust which happened.