Because they had a nutjob leader in North Korea that wanted to reunite the country under communism by force.
Yes, North Korea wanted to reunite their country that we and our WWII allies had divided. Also, the trusteeship plan fell apart (one reason being strong Korean opposition to it). After 1948, the two sides that we helped create claimed sovereignty over the whole peninsula.
So essentially, we helped create the mess in Korea that would eventually lead to the Korean War.
No we didn't essentially attempt an invasion, we did and it was a huge failure. JFK screwed the pooch on that one. Then the Cuban missile crisis took place with Kennedy rightfully putting his foot down on nuclear missiles being place in Cuba.
I completely agree with you about what JFK did in his attempted invasion of Cuba. It was a disaster.
So was the Soviet Union wrong to step in and support their ally by placing missiles in Cuba? Clearly, we had made and were making it clear that we were a direct threat to Cuba. We were not respecting Cuba's sovereignty. As you noted, we had invaded Cuba.
I think this clearly shows that we were in the wrong. We represented a threat to Cuba, who was an ally of the Soviet Union. The Soviet Union was perfectly justified to do what they did to support an ally who had just been invaded and was consistently being threatened by us (before and after the invasion).
Then JFK got played (privately at least) during the Cuban Missille Crisis. But that is another topic.
See any similarities as to whats going on in Ukraine?
Yes I do but not the similarities you claim. Russia doesn't respect Ukraine's sovereignty. Russia, led by Putin, invaded Ukraine. Not the other way around. Ukraine believes Russia to be a threat to them (clearly) and has sought support from other nations to balance this threat.
Yes, all of this does indeed sound very familiar to how we acted towards Cuba sixty years ago.