PRESIDENTS and not Congress should direct foreign policy, and that’s true for President Obama even after the walloping he and his party suffered in the midterm elections. Nevertheless, the end of the campaign and the change in the leadership of the Senate create opportunities for both the lameduck and the new Congress. They can authorize the war against the Islamic State, give Mr. Obama a needed nudge on military aid for Ukraine and weigh in on the endgame of negotiations with Iran.

Mr. Obama said at a news conference Wednesday that he would ask Congress for an Authorization for the Use of Military Force (AUMF) for the bombing campaign he launched in Iraq last August and later expanded to Syria. The president previously contended that he had legal authority to carry out the war under his constitutional powers and the AUMF laws Congress passed for al-Qaeda in 2001 and Iraq in 2002. With the midterms looming, many in both parties were happy to go along with that stretch of executive authority. But as Mr. Obama said following the election, “the world needs to know we are united behind this effort.”

In principle, it shouldn’t be hard to assemble a large bipartisan majority behind the goal of degrading the Islamic State. But the specific terms of the legislation could produce paralyzing dissension. Some will want to include provisions ruling out the use of U.S. ground troops in Syria or Iraq, spelling out goals for new Iraqi or Syrian governments or setting a time limit on the campaign. There is also the question — touched on by Mr. Obama at his news conference — of whether the new authorization should incorporate, revoke, or supplement those of a decade ago. In our view the most workable approach would be to keep the measure simple, allowing broad authority for the new military operations without congressional micromanagement.

Big bipartisan coalitions also can be built in Congress behind legislation providing Ukraine with weapons to deter further Russian aggression and applying more sanctions to Iran in the event it fails to agree to controls on its nuclear program by the end of this month. Only the Senate’s Democratic leadership and heavy White House lobbying has stopped such bills until now. A bipartisan measure approved by the Senate Foreign Relations committee in September, which provided $350 million for antitank weapons, drones and other munitions for Ukraine’s army, should be revived. Mr. Obama’s resistance to such aid at a time when Russia is again sending heavy weapons into eastern Ukraine makes no sense.

In the case of Iran, legislators should move more carefully. A U.S.-led coalition may succeed in completing a long-term deal with Iran on its nuclear program by a Nov. 24 deadline; another extension of an interim accord is also possible. Congress should avoid legislating new sanctions unless there is a breakdown in diplomacy. By the same token, Mr. Obama would be unwise to attempt to avoid seeking Congressional approval for a deal, or votes on the lifting of sanctions. If there is to be detente with Iran it should have — like the war against the Islamic State — broad and explicit political support.