The release last week of the U.S. Nuclear Posture Review (NPR) joins a series of recent and upcoming arms control events in which President Barack Obama is taking the lead. These also include this week's Nuclear Security Summit in Washington, the signing of the New START treaty with Russia, and the Nuclear Non-Proliferation Treaty (NPT) Review Conference, scheduled for May.

Despite the declared intention of addressing the problem of nuclear proliferation in a threat environment quite different from that of the Cold War, the changes to American nuclear policy reflected in the NPR are not revolutionary, although the document carries significant implications both for arms control and deterrence. Unfortunately, its explicit and implicit messages in both realms are worrisome, especially as they pertain to Iran's burgeoning nuclear program.

President Obama has a clear interest in creating the impression of significant arms control activity, so as to demonstrate his success in advancing the ambitious disarmament agenda presented last year in Prague - of a world free of nuclear weapons. What emerges from the NPR, however, threatens to make this lofty goal less achievable by distancing the world from effectively confronting one of the most pressing challenges it faces: the nuclear advances of Iran and North Korea.

The problems inherent in the NPR find expression primarily in its implicit messages. For example, the new policy reiterates a longstanding U.S. commitment that nuclear weapons will not be used against non-nuclear states that are party to the NPT, but adds the caveat that this "negative security assurance" will not apply to states that are not in compliance with their nonproliferation obligations and defy relevant UN Security Council resolutions.

The implication is that Iran and North Korea are examples of such states, and in statements explaining the NPR, President Obama has indeed emphasized the importance of this exemption as a means of isolating the two. The message to them is that noncompliance with the NPT makes them vulnerable to a possible U.S. nuclear attack.

At first glance, that may sound like a step forward, but one can reasonably ask: What is the real value of "nuclear isolation" in dealing with these stubborn actors?

Have efforts to stop Iran's nuclearization reached the point where they are to be "managed" by means of messages in a strategy document? Are we to believe, after the failure of all previous efforts to stop Iran's illegal activities, that it is this declared exemption that will finally break Iran's will?

The problematic nature of the NPR's isolation of non-compliant states is even sharper with respect to deterrence. On the one hand, the U.S. says it "may" target such exempted states with nuclear weapons under certain circumstances. On the other, one of the explicit goals of the NPR is a decided move away from reliance on nuclear weapons for U.S. defense of itself and its allies. So, which message dominates in the mind of an Iranian or North Korean decision maker?

That the NPR leaves this evaluation to be made by the offenders undermines U.S. deterrence. For a deterrent strategy to be effective, its threat needs to be both unambiguous and beyond doubt, not one that "may" be realized. As Nobel Prize-winning economist and strategist Thomas Schelling pointed out 50 years ago, "To say that one may act is to say that one may not, and to say this is to confess that one has kept the power of decision - that one is not committed."

The NPR claims that the fundamental role of U.S. nuclear weapons is deterrence, but the policy's goals threaten to work against each other.

If we were not facing the challenge of an Iran working steadfastly to acquire a military nuclear capability, we could probably welcome the NPR and the rest of this year's arms-control efforts, even if their achievements are small and slow in coming. However, confronted with the reality of a nuclearizing Iran and the serious repercussions its efforts carry not just for the Middle East, but also for President Obama's disarmament agenda itself - it would be a mistake to glorify these achievements at the expense of more determined efforts to curb Iran's ambitions.

Beyond the issue of misguided priorities, the NPR signifies further erosion of the official American commitment to prevent Iran from becoming a nuclear state, while raising troubling questions about the effectiveness of American deterrence in the face of Iranian aggressiveness both today and in the future.

Jonathan Schachter is a research fellow, and Emily B. Landau is a senior research associate at the Institute for National Security Studies, where she is also director of the Arms Control and Regional Security Project.