The purpose of nuclear weapons, or any weapon in the U.S. inventory, is to enhance the security of the United States. By declaring that the United States will not use nuclear weapons against states that do not possess nuclear weapons, President Barack Obama has enhanced the security of the country in two ways.
First, since the end of the Cold War, the primary threat from nuclear weapons has not been an all-out exchange with Russia. In fact, since the end of the Cold War, the United States and Russia have reduced their nuclear arsenals by nearly 80 percent. Rather, the danger is that nuclear weapons might fall into the wrong hands; specifically, into the hands of extreme regimes like Iran or North Korea, or of violent extremist groups like Al Qaeda. By establishing a policy that says the United States will not use or threaten to use nuclear weapons against non-nuclear states who are party to the Nuclear Nonproliferation Treaty, that gives countries an incentive to join and fulfill their obligations to the NPT. Moreover, it sends a signal to nuclear renegade nations that refuse to join the NPT—or who do not comply with their obligations under the NPT—that they could be targeted with nuclear weapons.
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