National security didn’t play heavily in the presidential election. But President Obama’s legacy, and the country’s future, will be shaped as much by the foreign policy and defense decisions he makes over the next four years as by those on the domestic side.

One of Mr. Obama’s singular contributions has been his vision of a world without nuclear weapons. It is a lofty goal that won’t be achieved in his second term, or maybe for years after that. But it offers a framework for reducing America’s stockpile and for arguing credibly that other countries should follow suit.

In 2010, Mr. Obama won Senate ratification of a treaty with Russia that makes modest cuts in deployed long-range nuclear weapons. It is time to pursue further reductions in those deployed systems, and to seek cuts in warheads held in reserve and in short-range nuclear weapons, where Moscow has a big advantage. Nuclear arms are one area in which the ability of Washington and Moscow to work together is essential. If Mr. Obama can draw the other nuclear powers, including China, Pakistan, India and Israel, into the discussions and persuade the Senate to ratify the Comprehensive Test Ban Treaty, so much the better.

The end of the campaign season might reduce the dangerous partisan posturing over Iran’s nuclear program. Iran and the International Atomic Energy Agency are to resume talks next month, but any diplomatic solution will at some point require direct negotiations between Washington and Tehran. Meanwhile, international sanctions, which have seriously damaged Iran’s economy, need to be rigorously enforced and strengthened.