President Barack Obama may be entering the lame-duck phase on domestic policy, but 2015 could be a defining year for his foreign policy. He faces several urgent tasks, notably three in the Middle East.

1. A nuclear deal with Iran.

A deal would end 36 years of tension between Washington and Tehran that has played out across the Middle East, contributing to setbacks in U.S. policies in Syria, Iraq, Lebanon, and beyond, as well as with the Palestinian Authority.

A deal would check a nuclear arms race in the world’s most volatile region, especially among the Persian Gulf sheikhdoms that feel most threatened by the Islamic Republic. Nuclear proliferation would draw the United States deeper into the region, not just because of energy issues and Israel.

A deal between Iran and the world’s six major powers would also help check a more deadly split between Sunnis and Shiites. Islam’s sectarian divide already affects a wider swath of territory than at any time since the faith was founded 14 centuries ago. A nuclear component would not help things.

Defusing tensions would be Mr. Obama’s biggest foreign policy accomplishment, dwarfing even his daring overture to Cuba. Iran has been been the nemesis of every U.S. president since its 1979 revolution. For the first time in decades, Washington and Tehran are nearing the same page, at the same time.

Hard as it may be, a nuclear deal is also the most possible of these three must-dos.

2. A hard press against Islamic State in Iraq.

Confronting or even just containing the world’s most aggressive extremist movement has to begin in Iraq, where Washington has a friendly government and may be able to rebuild the military.

The administration’s task includes helping Iraq retrieve Mosul, the country’s second-largest city, then pushing the extremist militia back toward the Syrian border. Since August, the U.S.-led coalition has launched more than 840 airstrikes to prevent Islamic State fighters from seizing territory beyond the third of Iraq it already controls.

But air power alone is not enough to force a retreat. And Iraq’s Shiite-led government has yet to convince enough Sunnis, through their tribal leaders, that it’s in their long-term interest to help. U.S. training will be critical to rebuilding a non-denominational military capable of expediting the ground campaign and holding Iraq together.

The odds of success anytime soon are slim. But the alternatives are much worse for the region and the rest of the world.

3. Salvaging Syria–and U.S. strategy.

The toughest of these three challenges is dealing with the multilayered war in Syria, which has produced more than 1,000 disparate fighting forces. The new U.S. plan is to create another rebel militia this spring by training and equipping 5,000 rebels annually for the next three years.

The new U.S.-backed fighters have two missions: defeating Islamic State extremists,with airpower support from a U.S.-led coalition; and pushing back the forces of PresidentBashar al-Assad, without foreign airpower, so that the regime is forced to negotiate.

The U.S.-backed rebels are grossly outnumbered: Mr. Assad’s army is estimated at 70,000 to 100,000, mainly air force, paramilitary, and special forces, and Hezbollah fighters from Lebanon. Islamic State is estimated to have more than 20,000 fighters in Syria. U.N. and Russian diplomatic initiatives surfaced last month, but neither has managed to win enough support to move forward. Syria urgently needs new, and bigger, thinking.

Syria is the strategic center of the Middle East. Its civil war has created the region’s most costly humanitarian disaster, with every country in the neighborhood destabilized–to different degrees–by spillover. Making it the second phase of the battle against Islamic State, to be addressed more robustly after Iraq, carries big costs.

But the list of pressing issues in the Middle East is long. Other areas that may demand Washington’s attention include Libya, a failed state that is rapidly crumbling into civil war. The costly NATO investment in ousting Moammar Gadhafi in 2011 looks almost like a waste of money.

Saudi Arabia may be closer to a transition; aging King Abdullah went to the hospital for medical tests last week. When Mr. Obama visited the kingdom last spring, oxygen tubes for the king–who reportedly smoked heavily for decades–were visible during their meeting.

Lebanon was floundering politically before Syrian refugees became a quarter of its population. Jordan, where the majority of the population is now Syrian, Iraqi, and Palestinian refugees, also feels vulnerable to regional instability. And the Palestinian-Israel conflict is a perennial, though little is likely to happen over the next year given other, more pressing issues.

Mr. Obama faces a packed agenda in the Middle East–again. But the stakes–ending two new wars and preventing a third–are higher this year.

The opinions expressed here are solely those of the author.

This article was first published in The Wall Street Journal's Washington Wire.