Cuba's justification, like North Korea's, is that they had the right to stockpile the weapons for their own protection. The reality of the situation was that the Soviet Union was using Cuba as a staging platform for nuclear warheads which would have the ability to strike targets deep inside the United States. This was in the days before the Intercontinental Ballistic Missile targeting systems were perfected to the point that such targeting could be accomplished from bases in the Soviet Union itself. Having them just 90 miles off the Florida coast also reduces the United States response time significantly.
As I was quite a young fellow in 1963, I can't claim to know the situation at the time. Looking at the timeframe, it would appear that, being a relative newcomer at the time, having only recently (1959) overthrown the government of Cuba and established a dictatorship with strong ties to the Soviet Union, there was likely considerable concern that Cuba was the shovel with which Kruschev intended to bury us.
My reason for bringing it up is that, if such an attempt had been made in Iraq, it would have been seen by many as providing justification for abandoning the sanctions against Saddam, and justified to some allowing him to proceed with his acquisition of WMD's. Assuming that we, in the United States, did not agree with this, our efforts to remove the threat would then be met with nuclear, chemical, or biological warfare agents unleashed against our troops, with potential for significant losses among Iraq's own people (from our invasion as well as from the broadcast of such agents). We also knew from Saddam's actions during the first gulf war that he had no qualms about lobbing a few of his missiles in the direction of Israel, even though they were not a part of the invasion force.
The distinction between offense and defense are sometimes blurry, particularly when talking about mutually assured destruction, or Defensive posturing by known billigerents.
Quod scripsi, scripsi.