Missiles were being placed in Cuba, because the USSR believed that they also need ICBM (intercontinental ballistic missiles) in striking range of the US, and placing them in Cuba (a pro-USSR country) was, in their minds, the right choice. This however, led to the Cuban missile crisis, which lasted for weeks in a over-tense strain between the relations of the US and USSR. However, at the end, the missiles were removed after the US was able to make a deal with the USSR, stating that the USSR would remove their missiles from Cuba if the US would remove her missile from Turkey.
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