U.S. Secretary of Defense Chuck Hagel has resigned. Many observers were surprised by the sudden departure, even though President Barack Obama had indicated that he would be shaking up his foreign policy team in the wake of the disastrous midterm elections held last month. Officially Hagel is leaving because the global security environment has changed since he took the job less than two years ago: Picked to oversee a military transition from war to peace, Hagel was instead presiding over new forms of engagement — tasks that demanded a different set of tools.

Just as important, however, is how poorly Hagel fit into the Obama foreign policy team. Reportedly a friend of the president and an adviser on security issues before he joined the Cabinet, Hagel never really penetrated the inner circle of the White House. In an administration that has kept a tight grip on foreign policy, that exclusion, along with a growing sense of a need to change his foreign policy team, meant that Hagel was the obvious choice to go. It also means, however, that this change is unlikely to cure whatever ails the Obama foreign policy team.

Hagel was nominated to replace Leon Panetta at the beginning of Obama's second term. It was thought that as a Republican senator and a former soldier — he would become the first enlisted man to serve as secretary of defense — he would win support from a hostile GOP and prove to be somewhat insulated from partisan attacks as he cut the budget during the Pentagon's transition from a wartime footing to peace. Instead, Hagel was roundly attacked by his former colleagues, who viewed his criticism of the Iraq War, along with his friendship with the president, as apostasy. He got through the Armed Services Committee on a straight party vote and was ultimately confirmed with just four Republican votes.

In office, Hagel was a less than commanding figure. To some degree, that reflected the unflattering contrast he made with predecessors such as Robert Gates, who enjoyed the full support of the military. For all his military bona fides — an infantry soldier who received two Purple Hearts — Hagel struggled to win over senior officers. That task was always going to be difficult as he resized the U.S. military from a wartime force to peacetime, drawing down from the conflicts in Iraq and Afghanistan, but the automatic cuts created by the sequester compounded the challenges. Any secretary of defense presiding over a shrinking budget will antagonize substantial constituencies in the Pentagon, no matter what their background.

Those difficulties were compounded by Hagel's exclusion from the most intimate White House counsels. The Obama administration has, more than any of its predecessors, tried to run foreign and security policy from the White House. Only someone with intimate ties to the president, a sure grasp of the issues and the appropriate policy responses to them would crack the inner circle. Despite his friendship with the president, Hagel reportedly came up short on the policy front. One indication of that shortcoming is the president's growing reliance on Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, for both advice and as the most visible Pentagon spokesperson on strategies to deal with crises.

The proliferating set of crisis — the deteriorating situation in Syria and Iraq, the rise of Islamic State, the Ebola outbreak in Africa, the standoff with Russia — transformed the nature of the secretary of defense's job. Of course, there are several critical national security jobs, but Obama is extremely close to National Security Advisor Susan Rice — with whom Hagel reportedly clashed in recent weeks — and Secretary of State John Kerry is deeply involved in the Iran nuclear negotiations. If the president felt a need to shake up his national security team, Hagel was the easy choice.

Unfortunately, Obama's national security troubles will not be solved by the replacement of one person, no matter how senior. The U.S. faces a larger set of perception problems. First, there is the commonly held view that the Obama administration is focused on events closer to home, working to withdraw from expensive overseas commitments and rebuild its domestic economy and society. Then, there is a belief that Washington is paralyzed, and that the president is a lame duck, unable to focus on serious foreign policy challenges, and lacking the stomach to respond to challengers like Russia's President Vladimir Putin or Syria's President Bashar Assad. Within Asia, China's rise and self-confidence are a direct challenge to Obama; for the most part, the U.S. president appears to be playing defense.

Replacing the secretary of defense will change none of those factors. It will, however, provide the Pentagon with a new head who is selected specifically to address new and emerging challenges. There is already a short list of candidates, but whoever ultimately gets the nod will have to go through what is likely to be a hostile Senate confirmation hearing. Having pledged to remain in his post until his replacement is named, Hagel may be secretary of defense for some time still, no matter how suited to this new reality he may be.