President Obama will sign the New START treaty when he meets Russian President Dmitry Medvedev in Prague on April 8, rounding out what has been a busy week on the nuclear weapons front.

On April 6, the administration announced the results of its year-long nuclear posture review. The review narrows the circumstances under which the United States might consider resort to nuclear weapons. While stating that the “fundamental purpose” of U.S. nuclear weapons is to deter nuclear attack on the United States, its forces, allies and friends, the review also said the United States will not use nuclear weapons against non-nuclear weapons states that are party to the Non-Proliferation Treaty (NPT) and in compliance with their NPT obligations. And, while previous administrations had maintained a degree of ambiguity about whether they would respond to a chemical or biological weapons attack with nuclear arms, the review makes explicit that this administration will not retaliate with nuclear weapons but instead launch a punishing response with conventional weapons.

The review did not go as far as some arms control proponents had hoped—it did not state that the “sole purpose” of U.S. nuclear weapons would be to deter a nuclear attack on the United States, its forces, allies or partners. But it nevertheless represents a significant step in the direction of reducing the role of nuclear weapons in U.S. policy.

New START, when ratified, will cut U.S. and Russian strategic nuclear warheads by some 30 percent—to the lowest levels in 40 years. The agreement will put in place a legal framework regulating U.S. and Russian strategic forces that includes a strong set of verification and transparency measures, through which we will know much more about Russian strategic forces than would be the case without the treaty. That kind of transparency builds confidence and promotes strategic stability. New START reportedly has a framework that will accommodate further reductions, which President Obama has called for and which the nuclear posture review endorsed.

The nuclear posture review and signing of New START set the stage nicely for two coming nuclear events. On April 12-13, Washington will host more than 40 global leaders for a summit on nuclear security. The focus will be on developing specific measures and an action plan to ensure, within four years, that all fissile material—highly-enriched uranium and plutonium—is kept under secure conditions. That’s important; no one wants this stuff leaking out to other countries or, the ultimate nightmare, to terrorist groups.

The nuclear posture review and New START also provide a good lead-in to the NPT review conference in May. U.S. diplomats will be able to show that the United States is living up to its NPT obligation to work toward disarmament. That will put them in a stronger position to press the conference to adopt more stringent measures to make it more difficult for other countries to acquire nuclear weapons. The nuclear posture review also will help the U.S. delegation at the review conference: by stating that the United States won’t use nuclear weapons against non-nuclear weapons states that are in compliance with the NPT, it aligns U.S. nuclear declaratory policy to bolster observance of the NPT.

All in all, not a bad week’s work.