He refused to give in to Russian demands for limits on missile defense and came away with an arms control treaty that, while modest, sets the stage for better relations. He got into high-profile scraps with the leaders of Israel and Afghanistan. And now he faces a critical test of whether he can forge a coalition to impose new sanctions on Iran.

Mr. Obama in recent days has backed down in his clash with President Hamid Karzai of Afghanistan. But during his news conference closing the nuclear meeting on Tuesday, he seemed to signal a renewed determination to reinsert himself into the Israeli-Palestinian dispute.

By describing the long-running conflict as a threat to American security, he effectively adopted the argument of Gen. David H. Petraeus, his Middle East commander, who recently warned that the region’s troubles created a dangerous environment for American troops stationed in nearby Iraq and elsewhere in the area. “It is a vital national security interest of the United States to reduce these conflicts because whether we like it or not, we remain a dominant military superpower,” he said. “And when conflicts break out, one way or another, we get pulled into them. And that ends up costing us significantly in terms of both blood and treasure.”

For most new presidents, foreign policy is a learning experience, and it can take months, if not years, to feel comfortable in the role of world leader. Advisers said Mr. Obama, like his predecessors, had grown more confident in managing international relations over time.

But he has learned hard lessons along the way about the limits of his powers of persuasion.

He has acknowledged that he underestimated, for instance, just how hard it would be to bring Israelis and Palestinians together, and his engagement with Iran yielded no more cooperation than Mr. Bush’s approach.