Attack

In your hastiness to match the Soviet Union in a show of force and end the threat quickly, you plan the attack too fast and do not put in the appropriate preparations. 

The attack plans were leaked to the Cuban government and they were waiting on the beach for the arrival of the troops. A massive slaughter ensued. 

This act of aggression touched off a series of alliances that pitted the U.S. and U.S.S.R. in a fight. That fight quickly became nuclear and all of humanity was wiped out by the massive amount of destruction and nuclear fallout.