Brokering an Israeli-Palestinian peace agreement - for recent American presidents, it’s been the holy grail of foreign policy. Unfortunately for President Obama, a comprehensive accord is just not in the cards for his second term.

It won’t be for lack of trying. But the inconvenient truth is that peace is impossible as long as Hamas retains its stranglehold over Gaza. Hamas is implacably committed to Israel’s destruction and well-positioned to torpedo a peace agreement. Even if Israel were to reach a perfect accord with the Palestinian Authority, Hamas could simply launch another round of rocket terrorism and the agreement would go up in smoke.

Secretary of State John F. Kerry reportedly wants to jump-start Israeli-Palestinian negotiations. But events will likely force him and his boss to focus elsewhere in the region — namely, on Syria and Iran.Syria is already proving problematic for Mr. Kerry. A “Friends of Syria” conference in Rome was supposed to get his first trip to the Middle East off on a positive note. But Syrian opposition leaders, upset by what they view as a lack of meaningful support from the U.S., threatened to boycott the event.

Mr. Kerry brought them to the table by promising to deliver more “nonlethal” aid directly to opposition forces, but that will not suffice to give the U.S. much influence over how things turn out in Syria. Far more efficacious would be arms aid — a program that would send light weapons to vetted non-Islamist factions of the opposition.

Mr. Kerry has favored such aid in the past. Now, he must convince President Obama that providing arms to friendly opposition forces is less risky than remaining on the sidelines while the opposition falls under the growing influence of Islamist extremists funded by Saudi Arabia and Qatar.

Already Syria’s meltdown is destabilizing the region. One million refugees have fled across the border, stressing the fragile economies of Jordan, Lebanon, Iraq and Turkey. Meanwhile, foreign Islamist extremists initially drawn to the carnage in Syria are starting to return home. They pose a growing threat to moderate regimes in Jordan and elsewhere. These spillover effects have overwhelmed U.S. containment efforts.

As for Iran, its accelerating uranium enrichment efforts will trigger a crisis later this year as it approaches Israeli Prime Minister Benjamin Netanyahu’s red line. The good news is that Mr. Obama and Mr. Netanyahu appear to have narrowed their differences over how to respond to this threat during the former’s visit to Israel last month. Both leaders publicly stressed Israel’s right to take action in self-defense. Some observers interpreted this as the president giving Israel a green light to launch a preventive strike against Iran’s nuclear program. More likely it was an effort to amp up the international pressure on Iran to make the concessions necessary to resolve this crisis through diplomatic means.

In the end, however, sanctions and diplomatic pressure may not be enough. Sanctions alone certainly failed to halt North Korea’s nuclear program, and Tehran seems no less committed than Pyongyang.

To dissuade Iran from continuing on its present course, the U.S. must present a credible threat that it will use military force if Tehran continues its diplomatic stalling tactics. It was no coincidence that Iran froze its nuclear program in 2003, after seeing the Bush administration take military action against the Taliban regime in Afghanistan and Saddam Hussein’s regime in Iraq. Libyan leader Moammar Gadhafi also agreed to give up his nuclear and chemical weapons programs because he feared he might be next.

Unfortunately, Tehran reversed course after it concluded that the United States was bogged down in Iraq. It resumed its nuclear efforts and hasn’t looked back.

To avoid a crisis, Iran must be convinced to reverse course once again. It should be required to give up its stockpile of medium and highly enriched uranium, close its Fordow uranium enrichment facility and accept enhanced international inspections of other nuclear installations. This will likely be the most important international challenge during Mr. Obama’s second term.

-James Phillips is the senior research fellow for Middle Eastern affairs at the Heritage Foundation (heritage.org).