Here’s a riddle that may determine the fate of President Obama’s Afghanistan policy: How do you rally international aid for Pakistan, a country hit by a flood of biblical proportions whose weak, unpopular leaders are unable to cope?

Flood waters are just starting to recede following a natural catastrophe that U.N. Secretary-General Ban Ki-moon labeled the worst the world has ever seen. “The magnitude of the problem,” Ban said, “it’s much beyond anybody’s imagination.”

Twenty percent of the country is submerged, and about 20 million are homeless. The country’s infrastructure is destroyed, including thousands of bridges, along with schools, hospitals, roads, dikes, dams, crops and livestock. Although the known death toll of 1,800 is relatively small, the devastation rivals the 2005 tsunami and the quake in Haiti combined, says Obama’s special envoy to Afghanistan and Pakistan, Richard Holbrooke.

As Pakistan’s economy teeters, Islamic militants are eager to take advantage. The army, already struggling with militants, is further stretched by flood relief. Meantime, Afghan Taliban still maintain havens in Pakistan’s tribal areas and dream of getting their hands on the country’s nuclear bombs.

Yet the global response has been painfully slow. “The world was not aware of the scale of the challenge,” Foreign Minister Makhdoom Shah Mahmood Qureshi told me last week in New York, where he was trying to rally assistance. “As people saw the footage, they woke up to the enormity.”

However, another big reason that donors have been reluctant is the worry that Pakistan’s government isn’t up to the job.

The Pakistani military is publicly pushing for a government shake-up, although it’s hard to see how that would make much difference. The flood has laid bare the massive inequality in Pakistan, which feeds the Islamist insurgency: The rural poor suffer, while the elite, including politicians and large-scale landowners, pay no taxes and corruption drains away foreign aid.

Yet the international community must find a way to help Pakistan. This is essential for obvious humanitarian reasons, but also to keep Pakistani militants from profiting from the tragedy and undermining the fight against al-Qaida and the Afghan Taliban.

The United States has taken the lead in relief and recovery efforts, mobilizing international donors, and sending helicopters and $345 million in aid. Some of the money has been diverted from the so-called Kerry-Lugar funds that Congress appropriated to help build up Pakistan’s pre-flood infrastructure. Billions more will be needed for reconstruction, and two international donor conferences are scheduled for this fall.

However, money can’t be raised unless donors believe it will go where it’s needed. Qureshi told me, “We’ve set up a new oversight council of 15 senior judges and civil servants with no political ax to grind” to oversee the money. That’s a good beginning, but more international oversight will be needed, too.

And as Holbrooke noted, Pakistanis will have to raise more money themselves, reforming their tax structure and taxing the rich. Some Pakistani economists suggest this could pay for a large share of rebuilding.

When I asked about raising taxes, Qureshi answered, “Why not?” and added, “Why doesn’t Chief Minister (Muhammad Shahbaz) Sharif in Punjab take the lead?” — referring to a member of the rich, landed family that leads the political opposition to Qureshi’s governing Pakistan People’s Party. But for tax reform to work, all the elites — including the ruling party — will have to pay their fair share.

And, finally, U.S. officials need to figure out a way to gain more leverage from America’s assistance to Pakistan. Anti-Americanism remains rampant, and most Pakistanis are unaware of U.S. aid. Meantime, the Pakistani military, which never needed America more, should be trying harder to deny Afghan Taliban safe haven.

“Pakistan is an ally that needs support now,” Qureshi said. He’s right. This staggering human tragedy requires a global response. And if Pakistan goes down, so does the effort to fight Islamic militancy in the region.

Yet Pakistan must look inward and make major structural changes if it is to survive and revive its economy. The world can’t help Pakistan if Pakistan won’t help itself.

— Trudy Rubin is a columnist and editorial-board member for the Philadelphia Inquirer. trubin@phillynews.com