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Nuclear weapons must be placed in a city or on a Carrier at the end of every turn (unless they are used to attack). Nuclear weapons may ignore zones of control and are invisible to enemy players. The Manhattan Project must be constructed to allow the construction of Nuclear weapons. When used on land, Nuclear weapons cause pollution in addition to destroying units. Therefore, they can be used against enemy naval units without the consequences of pollution.
The atomic bomb changed all the standards for measuring military power. The strongest cities and the most powerful ground forces could be erased from the map with the release of just one Nuclear bomb. Nuclear weapons could obliterate any target. The only viable defense against the Nuclear threat was to acquire one's own arsenal of Nuclear weapons as a deterrent. It is ironic to many that the development of the most devastating weapon of history was at least partly responsible for the longest period of relative world peace.