WASHINGTON — The weekend ended with the first tangible sign of a nuclear deal with Iran, after more than three decades of hostility. Then on Monday came the announcement that a conference will convene in January to try to broker an end to the civil war in Syria.

The success of either negotiation, both long sought by President Obama, is hardly assured — in fact the odds may be against them. But the two nearly simultaneous developments were vivid statements that diplomacy, the venerable but often-unsatisfying art of compromise, has once again become the centerpiece of American foreign policy.

At one level, the flurry of diplomatic activity reflects the definitive end of the post-Sept. 11 world, dominated by two major wars and a battle against Islamic terrorism that drew the United States into Afghanistan and still keeps its Predator drones flying over Pakistan and Yemen.

But it also reflects a broader scaling-back of the use of American muscle, not least in the Middle East, as well as a willingness to deal with foreign governments as they are rather than to push for new leaders that better embody American values. “Regime change,” in Iran or even Syria, is out; cutting deals with former adversaries is in.