What were the concerns for the Soviets after World war 2?

National security, after having suffered so much death and destruction during the German invasion in World War II. They were eager to align the nations of Eastern Europe alongside them in a communist bloc of nations as partners in a shared worldview and for mutual security.

The Soviets were also concerned about the massive power the United States possessed in the technology for atomic bombs. The Soviets made it a priority to develop their own atomic weaponry, and were able to do so by 1949.

And as much as Western nations feared an aggressive advance of communism in the world, spurred on by the Soviet Union, so also the Soviets were wary of what they saw as capitalist imperialism. They saw the USA and its western allies as a threat to their own convictions and way of life.