IN MIDEAST, U.S. POLICY MODELS BOG DOWN By Jay Solomon and Julian E. Barnes

WASHINGTON — For the better part of two years, U.S. military leaders have been looking for a middle ground in Syria between a full-out war and standing on the sidelines.

They have repeatedly thrown up their hands. Officials said their plans would either have a negligible effect or would almost certainly drag the U.S. into a prolonged conflict, potentially on the same side as al Qaeda-aligned forces battling the regime of Syria’s Bashar al-Assad. Meanwhile, the country’s civil war has claimed an estimated 100,000 lives.

In just a few years, the U.S. has executed a 180-degree strategic turn in the Mideast, from President George W. Bush’s muscular interventionism to President Barack Obama’s more backseat approach. That, according to some regional diplomats and experts, has disoriented Arab governments and Israel, who have become accustomed to extensive U.S. leadership in their region.

Two pillars have guided American foreign policy for a generation: the realist school that sought stability and tackled the world as it came, and the neoconservative trend that was willing to use force to realize American goals and values. Both are now struggling to provide answers for how the U.S. should respond to the Middle East’s continuing upheaval.

“I would challenge anyone today to tell us what our Mideast strategy is,” said retired Gen. James Mattis, the former top military commander in the Middle East. “The realist and interventionist schools of foreign-policy thought are not established on any strategic basis. The crises are blowing away an intellectual fog, revealing there is nothing there.”

The Bush administration hewed to a neoconservative strategy that sought to fundamentally reshape the region through the wars in Iraq and Afghanistan.

President Obama pledged in 2012 to “pivot” the U.S. away from the Middle East as part of his broader strategy to wind down America’s wars over the past decade. Regional diplomats and scholars say that has resulted in not only a precipitous military pullback, but also a less-aggressive diplomatic and economic approach. The Obama administration has publicly doubted the U.S.’s ability to dictate events in the region, said current and former U.S. officials.

Regimes considered U.S. allies seem increasingly disinclined to follow Washington’s advice and are suspicious of its preference for democracies. Meanwhile, American voters and their military, exhausted by two long wars, are hesitant to intervene. Among the fallout has been gruesome sectarian violence in Syria and a brutal crackdown by the Egyptian military on the Muslim Brotherhood.

“The state structure in the Middle East has been quickly changing and the boundaries are shifting in Iraq and Syria. And then you have this sudden withdrawal of the U.S., which was the stabilizing force in the region,” said Vali Nasr, a former Obama administration official now dean of John Hopkins’s School of Advanced International Studies.

Diplomats and scholars sympathetic to Mr. Obama’s quandary note he is facing a region in the throes of epochal change armed with constrained U.S. resources. Post-Iraq, American politicians and voters have also shown little interest in another foreign entanglement.

Still, there is growing concern in Washington and Europe that eventually the West will have to intervene more forcefully in the Mideast, most likely Syria, and at a time when the costs to the U.S. would likely be higher.

“Obama’s presiding over a slow, incremental American disengagement. People can’t believe it’s happening, but it’s happening,” said Steve Clemons, a foreign policy expert at the New America Foundation, a liberal Washington think tank.

Mr. Obama has acknowledged that Egypt and Syria are complex challenges. The White House and State Department say they have been engaged in developing international responses to end these conflicts. They also say any kind of military intervention inside Syria risks further inflaming the Mideast and Muslim world.

“What we would like to see moving forward is a continued effort on the part of the international community to work together to address some of these problems,” said White House spokesman Josh Earnest on Thursday. “But we would also like to see in a lot of these situations the leaders of these countries to respect the basic human rights of the people that they govern.”

Mr. Obama’s response to the twin crises in Egypt and Syria illustrate both the U.S.’s scaled-back ambitions for altering the Mideast, and how other countries have been trying to fill the leadership void, often in ways contrary to American interests.

The Obama administration, while cautious in responding to the Egyptian revolution in 2011, ultimately embraced the fall of dictator Hosni Mubarak and pledged to strongly back Cairo’s new democratic government. The U.S. and Europe talked of a new Marshall Plan for the Middle East with Egypt, the Arab world’s most populous nation, as its pillar.

More than two years on, total U.S. and European economic assistance to Egypt is estimated to be below $1 billion. U.S. officials say punitive actions taken by the Egyptian government against American nongovernmental organizations have constrained aid, as well as Cairo’s reluctance to execute economic reforms championed by the International Monetary Fund.

Oil-rich Arab states, in particular Qatar, Saudi Arabia and the United Arab Emirates, have pledged or invested more than $20 billion in Egyptian governments over this time. And in recent weeks, Riyadh and Abu Dhabi have publicly challenged the Obama administration’s calls for the Egyptian military to end its crackdown on the Muslim Brotherhood, vowing to write even more checks if needed.

“We think America is thwarting our regional policy toward Egypt,” said a senior Arab official. “We fundamentally disagree on the role of the Muslim Brotherhood.”

Syria has emerged as an even trickier issue for the Obama administration and its European and Arab allies.

When the uprising against Syrian President Bashar al-Assad broke out in early 2011, many U.S. officials saw his overthrow as serving a crippling blow to Iran, Washington’s chief antagonist in the region.

But while the Pentagon and White House have continued to debate what steps to take in Syria, Iran and Russia have mobilized to prop up Mr. Assad. In recent months, Tehran has facilitated the flow of thousands of Shiite fighters from Lebanon and Iraq to join the fight with Syrian forces, according to U.S. and Arab officials.

As the Mideast crisis deepens, American foreign policy strategists, both neoconservative and realist, are grappling for a new lens through which to stabilize the region.

Michael Rubin, a neoconservative scholar at the American Enterprise Institute, said the competing schools are frequently treated in simplistic ways that don’t recognize the vibrancy of the debate within them.

Mr. Rubin favored intervening in Syria, two years ago. But now he says the prudent course must be to wait and see which of the two sides prevails. In the case of Egypt, Mr. Rubin favors siding with the Egyptian military.

Many realists say, however, they back Mr. Obama’s current policy, despite the violence. They question whether the U.S., even at the height of its power, could influence a crisis like Egypt where the foundations of the state are breaking down.

“Even if the U.S. was at the top of its game, it would have limited influence on what’s happening,” said Charles Kupchan, a professor of international relations at Georgetown University. “It’s passing through an historical intersection in which there are social and political changes taking place.”

— Colleen McCain Nelson contributed to this article.