U.S. Navy Secretary John Phelan is leaving his post, the Pentagon announced Wednesday, marking the first departure of a military service chief during the second term of U.S. President Donald Trump.

The Pentagon did not provide a reason for the abrupt exit. Phelan’s departure comes at a critical moment, as the Navy is actively enforcing a blockade of Iranian ports and targeting vessels linked to Tehran during a fragile ceasefire in the ongoing conflict.

Undersecretary Hung Cao will assume the role of acting Navy secretary. Cao, a combat veteran with 25 years of service, previously ran for Congress and the U.S. Senate in Virginia.

The move is the latest in a series of leadership changes at the Pentagon under Defense Secretary Pete Hegseth, who has overseen multiple high-level dismissals since taking office.

In recent weeks, Hegseth removed Army Chief of Staff Randy George and has also dismissed several senior military officials. Earlier actions included the removal of Adm. Lisa Franchetti, the Navy’s top uniformed officer, and Gen. Jim Slife, the Air Force’s vice chief.

President Trump also dismissed Gen. Charles Q. Brown Jr., further underscoring the sweeping changes within the U.S. military’s top leadership ranks.

Phelan’s exit adds to ongoing uncertainty within the Defense Department as it navigates both internal restructuring and heightened global tensions.