The White House has not settled on a candidate to replace Defense Secretary Chuck Hagel. (Chip Somodevilla/Getty Images)

When Chuck Hagel took charge at the Pentagon in early 2013, Syria’s civil war looked like a poisonous morass: 60,000 people had died, an array of armed factions was doing battle on the ground, and widening instability had U.S. officials scrambling to avoid being dragged into another war in the Middle East.

As Hagel prepares to step down under pressure almost two years later, the picture is largely the same — except that 200,000 Syrians have now died and the United States is in the thick of an expanding military campaign in both Syria and neighboring Iraq.

White House officials, making the former Nebraska senator’s hastily arranged resignation announcement Monday, did not publicly fault Hagel for his management of the military response to a security crisis now consuming much of the Middle East.

But they did acknowledge that Hagel, selected in part for his experience as an infantryman in Vietnam, was not the wartime secretary they required.

Since this summer, when Islamic State fighters conquered a full third of Syria and Iraq, U.S. warplanes have conducted nearly 870 strikes on militant targets in those countries. Obama, who in 2011 hailed an end to the last U.S. war in Iraq, has reluctantly sent hundreds of service members back to Iraq to help the country shake the militant threat.

The White House has not settled on a candidate to replace Hagel, and one of the leading contenders for the job, Michèle Flournoy, who served as undersecretary of defense for policy earlier in the Obama administration, indicated Monday that she is not interested in being considered. Several other top contenders, including former official Ashton Carter, have deep experience at the Pentagon.

No matter who Obama selects as Hagel’s successor, the next defense secretary is likely to struggle, as Hagel did, to contain the instability rippling across the Middle East, given the potency of the Islamic State, the weakness of U.S. partners on the ground and the limits White House policymakers have imposed on U.S. involvement.

“The challenges don’t change because the secretary of defense has changed,” said Robert Ford, the last U.S. ambassador to Syria.

Ford said the Obama administration would need to ensure that Iraq’s fragile coalition government builds lasting inroads with Sunni Muslims, some of whom are now supporting the Islamic State, even while it seeks to rebuild a broken, undermanned military.

In Syria, he said, officials must find a way to reconcile the White House’s goal of weakening the brutal Islamist movement with its aversion to taking direct action against Syrian President Bashar al-Assad, whose war with opposition fighters has given rise to the group.

Despite months of U.S. and allied airstrikes, Iraqi forces and Shiite militiamen have not been able to dislodge the well-armed Islamist fighters from most of the areas they control across the country’s north and west.

In Syria, U.S. bombs have taken a toll on the Islamic State but Assad remains comfortably in power. There are few signs the White House will soon alter its approach to the conflict.

Defense Secretary Chuck Hagel announced his resignation, stepping down under pressure from the White House. Here's a look at how he responded to some of the biggest defense issues during his tenure. (Julie Percha, Jackie Kucinich and Rebecca Schatz/The Washington Post)

“We don’t foresee any major changes to the strategy against ISIL as a result of the secretary’s resignation,” said a senior U.S. defense official, using an alternative name for the Islamic State. “He helped craft it. He’s been in on the decisions. That said, that doesn’t mean the strategy won’t change over time,” he said, speaking on the condition of anonymity to discuss policy deliberations.

Officials say Hagel, who is likely to remain in his post until a successor can be confirmed next year, was not particularly hands-on regarding Iraq and Syria.

While the day-to-day military operations are managed by U.S. Central Command, a new secretary might reclaim some of the military voice that an introverted Hagel, known for rarely speaking up in meetings among senior Obama advisers, ceded to Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff.

Dempsey, despite Obama’s insistence that U.S. troops will not return to combat in Iraq, has indicated that U.S. troops may eventually need to get closer to the front lines to help Iraqi forces repel the Islamic State.

The next defense secretary will also have to contend with a sometimes-tense relationship with the White House. Both of Hagel’s predecessors, Leon Panetta and Robert Gates, have criticized Obama’s handling of national security matters since leaving office and have complained of White House micromanagement of the military.

“Whoever the new secretary of defense is, they’re probably going to want to discuss with the leadership of the National Security Council the scope of freedom for decision-making at the Pentagon,” a former U.S. official said, referring to what he described as the White House’s desire to tightly control national security policy.

“Pentagon and State have to adjust to that,” the official said, speaking on the condition of anonymity to discuss internal administration dynamics. “If you can’t adjust, you leave.”

The next Pentagon boss will also have to find a way to build support among Americans for a campaign that U.S. commanders are warning will last for years and that may not make concrete progress unless Iraqis can set sectarian politics aside and Syria’s quarrelsome political opposition can come together.

“That’s a strategy that doesn’t work in American public debate, because it has words like ‘containment,’ ‘patience’ and ‘long-term,’ ” said Jeremy Shapiro, a former State Department official who worked on Middle East policy. “It’s a strategy that can work, but it’s not a strategy of victory, if you know what I mean.”

Julie Tate contributed to this report.