After Kennedy ordered the blockade, the Russians still had ships on the way to Cuba.
The ships were not far from the blockade, giving every indication that they were going to run the blockade
The ships were being escorted by a submarine out ahead and in between them.
If you fire on the sub, you may start World War III. If you let them through the blockade, then your blockade was meaningless.
DECISION: What do you do about the approaching Russian Naval vessels?
Click here to see what Kennedy did