For MAD to succeed, each side needed to be able to retaliate, even after it had suffered a surprise attack. Submarines now played a crucial role -- as mobile launch platforms for nuclear missiles. Both sides practiced civil defense against nuclear attack, but it was generally believed that it was impossible to defend against nuclear weapons.

Even short of total war, nuclear deterrence carried its own dangers. In 1966 over the coast of Spain, a U.S. bomber collided in mid-air with a tanker aircraft. As the planes crashed, four hydrogen bombs were scattered over the coast. Three hit the ground and were recovered. One fell into the sea and was lost for 80 days. Such "Broken Arrow" incidents were growing, as both sides increased their nuclear arsenals.
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