An activist foreign policy is hard to sustain on a shrinking material base. In the ’70s Leonid I. Brezhnev rejected the arms-control concessions that American presidents asked of him. He was building more and bigger missiles; the Americans were not. In diplomacy, you need something to bargain with. Retrenchment typically gives you less of it.

The Obama administration has built new leverage in one crucial case. A tough American-led sanctions regime has gotten Iran’s attention. Elsewhere, Mr. Obama and Mr. Kerry count too much on negotiation to achieve their goals. Ruling out an assertive role in Syria’s civil war reduced American influence over both government and rebels. No surprise, then, that the “Geneva II” peace conference has deadlocked.

Resources will test Mr. Obama’s retrenchment strategy in other regions. In 2011, with the Pentagon budget under pressure, the president pledged that spending cuts would not affect American forces in the Pacific. The vow was a crucial element of his “pivot” to East Asia, and the region’s governments and publics will note carefully how fully it is kept.

Delivering won’t be easy. The administration has had trouble fulfilling smaller commitments. Early in the Arab Spring, Mr. Obama sketched plans to support democratic evolution in the Middle East. Aides say he is “disappointed” by the poor follow-through on his aid pledges. (But not disappointed enough to push hard to keep them.)

History has another sobering lesson: retrenchment always strengthens Congress’s role in foreign policy. Mr. Kissinger raged against legislation that blocked most-favored-nation trading status for the Soviet Union, and covert aid for rebels in Angola. Eisenhower ridiculed Congressional proposals to increase the defense budget.

Mr. Obama faces similar pressure. In his recent State of the Union address, he threatened to veto any bill that imposed new sanctions on Iran while talks were underway. Let’s hope he also knows how badly previous administrations handled similar challenges. They were quick to label Congressional critics as extremists or tools of special interests, slow to recognize when the critics had a strong bipartisan base. Mr. Kissinger called the Angola ban a sign of McGovernite Democrats’ nihilism. In fact, Republican senators from Jacob K. Javits to Jesse Helms supported it. Even Eisenhower’s complaint about the influence of the “military-industrial complex” was one volley in a debate he knew he was losing. Those who pushed for greater activism, like John F. Kennedy, already had won.

So what’s a retrenchment president to do? If Eisenhower, Nixon and Bush — our best prepared foreign-policy presidents — found it hard to manage a downsized strategy, Mr. Obama should expect the same. He won’t abandon retrenchment, nor does he need to. The domestic foundations of American power do need shoring up. But he needs to tend to its international foundations as well.