What effect do nuclear weapons have on interstate conflict behavior? Do nuclear weapons bolster the defense, deterring aggression and making states more secure? Or do nuclear weapons embolden the states that possess them, leading to conflicts that would not have occurred in their absence? Do nuclear weapons have different effects on conflict depending on the nuclear capabilities of the adversary? These questions, central to policy debates during the Cold War, have only grown in importance in the last few decades with the emergence of new regional nuclear powers such as Pakistan, India, and North Korea. Today, debates over U.S. policy toward the Iranian nuclear program largely hinge on differing opinions about the answers to these questions, as pessimists worry nuclear weapons will increase Iran’s belligerence (e.g., Kroenig 2012), while optimists argue nuclear weapons will either dampen Iranian aggression (as Iran grows more secure) or have no effect on Iranian behavior due to the U.S. and Israel’s ability to deter Iran with nuclear weapons of their own (e.g., Waltz 2012).