Why did the US have to counter the Soviet build up of missiles in Cuba?

During this period the USA and its friends in europe had lietraly 5 miles from the soviet union or Russian territory. at the time Russian did not have an short flight response in place to attack the usa. this ment we could lunch a nuclear strike at the Russians and Russia culd not responed against the usa for sevaral hours and the usa could likely mount a good defense and prevent an attack. so putting missiles in cuba seemed like it would "balance" the detrent.