OF ALL the international crises facing President Obama in his final two years in office, how to cope with a burning Iraq and disintegrating Syria may be the most daunting. Syria, especially, is facing the most serious humanitarian crisis in the world today.

Washington’s current debate is focused, understandably, on the immediate geopolitical risk — an expanding ISIS caliphate in northern Syria and western Iraq. The Obama team seeks to contain ISIS in the near term through US-led air strikes and to defeat it over time by destroying its base in Syria. But this will be a war whose success or failure will be written not in months but years. While Obama has worked out a potentially effective Iraq policy, he has been singularly indecisive on Syria at every important moment since its revolution began in 2011.

And Syria is his next challenge. That tortured country is nearing a catastrophic humanitarian meltdown and could even disappear as an integral nation state. Syrian President Bashar al Assad’s vicious assaults on Syria’s civilian population for three years now have destroyed much of the country’s infrastructure.

As a result, its 22 million people face an increasingly desperate situation. More than 200,000 Syrians have been killed. Half the population — over 11 million people — is now homeless. No other country is confronting a crisis of such magnitude. With more than a million Syrian refugees in weak and unstable Lebanon and a million more in besieged Jordan, a regional war far worse than anything to date is a growing possibility.

David Miliband, the former British foreign secretary and current president of the International Rescue Committee that is supplying urgently needed humanitarian assistance to Syrian refugees, describes it to me as “the greatest humanitarian crisis of this century.” He warned that while “more aid is getting through, conditions of life for Syrians are getting worse.” And now even the supply of aid is in question. The World Food Program suspended some food deliveries to refugees this week for lack of funds.

What should the United States and other countries do to help? Miliband offers three sensible ideas. First, he advocates a massive increase in aid to sustain Jordan and Lebanon. Second, he urges “cross border” international relief operations into rebel-held territory to reach the most vulnerable people. This will be exceedingly dangerous, but Miliband believes millions of Syrians are in need, especially in Aleppo, Syria’s largest city. Third, Miliband suggests the United States and other powers push to broker local cease-fires, open more reliable “cross line” corridors, and speed humanitarian aid to the people who most need it. A big problem is that Russia and Iran have refused to push Assad to stop his vicious attacks against Syrian civilians.

Since Shiite and Sunni forces first collided in 2011 to spark Syria’s civil war, many have advocated the creation of humanitarian exclusion zones inside Syria near the Turkish and Jordanian borders to protect civilians fleeing the fighting. These areas would need to be protected by the United States and other militaries to shield civilians seeking shelter from the rampaging Syrian government and some of the most radical rebel forces. Security risks and high financial costs have made this idea unsellable in Washington. But is it right to stand by as the slaughter continues?

We faced a similarly impossible set of options two decades ago during the Clinton administration as the Bosnian war took tens of thousands of lives. America and Europe temporized until we were shamed into action following the murder of 8,000 Muslim men and boys by Bosnian Serb forces at the killing grounds of Srebrenica in July 1995.

Syria’s civil war will almost certainly worsen as 2015 approaches. Obama needs to act with much greater energy and conviction to help millions of Syrians escape their increasingly unbearable national nightmare.