Was the Soviet Union justified in establishing a sphere of influence in Eastern Europe in order to achieve national security?
This is, of course, a matter of opinion. I will provide three different arguments and you can choose which you agree with.
On the one hand, we can argue that there was no way to justify what the Soviet Union did. In this view, the Soviets had nothing to fear from the Western allies. There was no way that the US, France, and Britain had any intention of invading the USSR to overthrow their communist system. The Western allies would have been happy to live in peace with the Soviets, so the Soviets had no need to create a buffer zone.
A second argument, however, would say that the Soviets could not rely on the good intentions of the Western allies. The Soviets had to worry about what the Western allies could do, not what they probably would do. Russia had been invaded by France in 1812 and the USSR by Germany in 1941. The Soviets had ample reason to fear invasion from the West and had no choice but to protect themselves against this possibility.
A third argument is somewhat of a compromise position. In this view, the Soviets were right to worry about invasion from the West, but should have been less heavy-handed in creating their sphere of influence. The Soviets should have used economic and diplomatic approaches to creating a sphere of influence instead of imposing their will on their satellite states by force. This would have protected Soviet interests without infringing on the rights of the countries in their buffer zone.
Which of these three arguments makes the most sense to you?