Kennedy was most effective during the Cuban Missile Crisis. This crisis resulted from the Soviet Union putting missiles in Cuba after the US put missiles in Turkey. The fear that the Soviet Union might attack the US with these missiles resulted in massive fear among American citizens.
However, over the course of thirteen days in October of 1962, Kennedy was able to navigate this crisis and organized a peaceful solution to the problem. The US agreed to take their missiles out of Turkey and the Soviet Union would remove their missiles from Cuba. If Kennedy did not negotiate a deal like this, the world would have experienced all out nuclear warfare for the first time.
