Soviet Union never was on defensive side. From the very beginning they've tried to conquer most of their neighbors, they've attacked Poland together with Hitler and Finland in 1940.
Even in 1945 they've tried to annex part of Persia and wrestle Bosfor Straight control from Turkey.
It has been the historical norm for great power's to exert control over a sphere of influence. That is exactly what the United States did in the 19th century in the Western hemisphere through the Monroe Doctrine. The cases you raise fall into that category.
In fact there is an obvious parallel between the USSR's attempt to gain access to the Turkish Straits and the United State's repeated interventions to build and control the Panama Canal. The only difference is that while the United States was able to dominate its sphere of influence, the Soviet Union's attempt to intimidate Turkey failed. Instead of gaining access to a vital strategic waterway, it led to the Truman Doctrine. Don't forget that one of the pivotal events of the Cold War - the Cuban missile crisis - was based on the same asymmetry. The United States had Jupiter missiles in Italy and Turkey, but would not accept Soviet missiles in Cuba, a country that it had tried and failed to invade.
Before the end of the Second World War the United States had set its sights on something far more ambitious than a regional sphere of interest: global primacy. The Soviet Union never had the capability to do that, or considered it a realistic goal. It was bunkered in a traditional land empire.
When I say that the Soviet Union was on the 'defensive', I mean that in the context of its bipolar confrontation with the United States, not that it never engaged in aggression. Obviously it did, often brutally.
I think that's a very unhelpful way of putting the matter.
There is no direct confrontation standing at the origin of the conflict. Hence it being a 'Cold' War. It emerged from an accumulation of strategic moves over time. Each power worked to advance its interests, while watching other powers (namely Germany, Japan, and the British Empire) fade. At some point, they recognised that they were in direct geopolitical competition. When they did, it was clear that the Soviet Union was the lesser power. Its actions reflected that reality.
If you did want to identify an 'originary' moment to the conflict - which again, I think is an unhelpful way of approaching the subject - then it would have to be the US intervention in the Russian Civil War against the Bolsheviks.
Even in 1945 they've tried to annex part of Persia and wrestle Bosfor Straight control from Turkey.