US President Barack Obama puts his hand on the shoulder of US Defense Secretary Chuck Hagel during a press conference to announce Hagel's departure at the White House on November 24, 2014 in Washington, DC. Hagel will remain at his post until a successor is named and confirmed by the Senate. (Mandel Ngan/AFP/Getty Images)

IF THE resignation of Defense Secretary Chuck Hagel augurs a move by President Obama to shake up his national security team and reconsider his strategy in crisis areas such as Syria and Ukraine, then it will be welcomed. So far, there’s not much sign of it. Mr. Hagel has been a weak leader at the Pentagon who, at least in public, has been less of a force in policy discussions than some of the generals who report to him. But his thinly disguised dismissal came after reports that he had raised sensible questions about Mr. Obama’s overly constrained approach to fighting the Islamic State.

The president turned to Mr. Hagel, a friend from the Senate, after the 2012 election seemingly because he wished for a Pentagon chief who would willingly execute his second-term priorities of withdrawing from Afghanistan, avoiding other foreign military engagements and implementing big budget cuts. This agenda of retrenchment, however, was overtaken by the Islamic State’s rise and Russia’s invasion of Ukraine.

Like the two defense secretaries who preceded him in Mr. Obama’s Cabinet, Mr. Hagel found himself caught between an insular White House team and military commanders who chafed at what they see as both micromanagement and the absence of a workable strategy. By this fall, according to the New York Times, Mr. Hagel was telling the president what he least wished to hear: that U.S. policy in Syria was inadequate because of its failure to address the regime of Bashar al-Assad.

Such views echo those of present and former senior U.S. officials who question how the United States can look to Syrian rebel forces to fight the Islamic State without defending them from the stepped-up attacks of the Assad regime. And that’s not the only hole in Mr. Obama’s strategy: Commanders including the Joint Chiefs of Staff chairman, Gen. Martin Dempsey, have suggested that the campaign in Iraq will eventually require the limited use of U.S. ground forces — something the White House strongly resists. As Mr. Obama’s first defense secretary, Robert Gates, recently put it, “When you . . . deny the military the authorities they require to achieve the objective, you leave them with a great sense of frustration.”

These differences mean that Mr. Obama’s choice for a new defense secretary will be particularly important. What’s needed is a seasoned, independent policymaker with a genuine command of military affairs who is able to win the respect of both the generals and the White House. Former undersecretary of defense Michèle Flournoy, whose knowledge of Pentagon budget arcana as well as grand strategy is formidable, would be an excellent choice.

To work with such a strong leader, the president must accept that his strategy as well as his staff are in need of rethinking. One encouraging sign came Friday, when the administration let it be known that it will allow U.S. forces in Afghanistan to conduct offensive combat missions, including air support for Afghan troops, after this year — an adjustment essential to preserving the Afghan government and army. Mr. Obama needs to choose advisers who will offer him fresh thinking about national security challenges — and be ready to set aside his own fixed ideas.