WASHINGTON — Ambassador Tina Kaidanow, a longtime State Department official who joined the Pentagon in 2018, has left her position as senior adviser for international cooperation, Defense News has learned.

Her departure represents the fifth high-level official at the Defense Department to either exit or announce plans to leave the Pentagon in just the last week.

Kaidanow stepped down from her role on Dec. 16, department spokesman Lt. Col. Michael Andrews told Defense News. Michael Vaccaro, the department’s director for international armaments cooperation, will serve as the acting senior adviser.

“The department remains committed to the development and implementation of international cooperative programs and defense exportability efforts to foster cooperation with U.S. allies on research, development, production and support of weapons systems and related equipment,” Andrews said in a statement. “The department will not provide anything further on this personnel matter.”

Kaidanow was the first to lead the international cooperation office, which was created under a Pentagon reorganization to ensure “mutually beneficial international cooperative R&D programs consistent with national security considerations.” She worked closely with Under Secretary of Defense for Acquisition and Sustainment Ellen Lord, as well as Lt. Gen. Charles Hooper, the head of the Defense Security Cooperation Agency, on growing weapon sales with foreign partners and allies.

Kaidanow served as the principal deputy assistant secretary of the Bureau of Political-Military Affairs at the U.S. State Department — and as its acting assistant secretary for political-military affairs — between 2016 and her joining the Pentagon. In that role, she was the State Department’s point person on arms transfer issues, as well as a key point person in communications between the Defense and State departments. She also was instrumental in reforms to the Conventional Arms Transfer policy, part of the Trump administration’s push to increase American arms sales abroad.

Kaidanow’s departure is the fifth notable exit from the department in just the last week.

On Dec. 12, it was announced that Randall Schriver, the Pentagon’s top Asia policy official, would be leaving shortly. One day later, on Dec. 13, Jimmy Stewart, who served as the acting undersecretary of defense for personnel and readiness since October 2018, announced his retirement from federal service. And on Tuesday, Defense News reported that Steven Walker, who served as the 21st head of the Defense Advanced Research Projects Agency, will leave the position on Jan. 10 for an industry job. And earlier today the department confirmed that Kari Bingen, principal deputy undersecretary of defense for intelligence, will also resign as of Jan. 10.

The exodus means there are now five more top jobs among an existing number of vacancies inside the department. Just inside the policy shop, David Trachtenberg, the deputy undersecretary of defense for policy, retired in July, while Robert Karem departed as the assistant defense secretary for international security affairs in October 2018; both those roles are filled by acting individuals. Currently, six of the 21 deputy assistant secretary of defense for policy positions are unfilled.

Matt Donovan, the undersecretary of the Air Force, has been temporarily moved into the spot vacated by Stewart, creating another hole. Other top positions filled by acting officials include the chief management officer and comptroller roles, as well as the secretary of the Navy.

In August, then-newly appointed Defense Secretary Mark Esper acknowledged that there needs to be “stable leadership” among civilian leaders in the department, and expressed his belief that the vacancies would be filled. On Wednesday, the Senate Armed Services Committee’s ranking member, Sen. Jack Reed, D-R.I., warned that the practical effect is that with fewer career professionals and experts to draw on, “you’re not anticipating problems and you’re not as well-prepared to respond to problems. So this is a very difficult problem.”

Joe Gould in Washington contributed to this story.