Why was it such a big deal when the U. S. discovered missiles in Cuba?

TIA

It was a big deal because the missiles were being built by the Soviet Union, our enemies at the time.. and when we discovered them, we realized that had a plan to use them on the U.S... the discovery of missiles in Cuba let to the prevention of nuclear war agreement.. which meant if they bombed us, we would bomb them and neither the Soviet Union nor the us wanted that.. so they agreed not to use their nuclear weapons