What caused the Cuban missile crisis?

This depends
on how you mean the
question.

The most immediate cause of the Cuban Missile

Crisis was the fact that US U-2 spy planes detected Soviet missiles in Cuba.  The US
felt that
this was too much of a threat to US soil and therefore Pres.
Kennedy started to take actions to
force the Soviets to remove the
missiles.

If you take another step back, the
cause of the
crisis was the fact that the Soviet Union did not have missiles that could hit
the
US mainland while the United States did have missiles that could hit the
Soviet Union.  This
imbalance made the Soviets uneasy and led them to try to
even it out by putting missiles in
Cuba.