WASHINGTON (AP) — Defense Secretary Lloyd Austin warned that troop readiness and retention is at risk as the Army's chief stepped down Friday, leaving the military's two ground combat forces without Senate-confirmed leaders for the first time in history.

Speaking during a ceremony at Joint Base Myer-Henderson Hall, Austin said the Senate's failure to confirm the services' new leaders is disruptive to the force and could impact relationships with allies and partners around the globe.

The confirmation of the next Army chief and Marine commandant are among more than 300 military nominations stalled by Alabama Sen. Tommy Tuberville, a Republican, over the Pentagon's policy to pay for travel when a service member has to go out of state to get an abortion or other reproductive care.

"Today for the first time in the history of the Department of Defense, two of our services will be operating without Senate confirmed leadership," said Austin. "Great teams need great leaders, and that's central to maintaining the full might of the most lethal fighting force on earth."

Army Gen. James McConville is retiring and Gen. Randy George, the current vice chief has been nominated to become the next chief of the service. On Friday, George became the acting chief. Similarly, Marine Gen. Eric Smith has been nominated to be the next commandant, but is serving in an acting capacity now because he hasn't been confirmed.

Both can serve as "acting" chiefs, but can do nothing that would presume confirmation. As a result, they can't move into the main residences or offices, or issue formal planning guidance, which is traditional for a new leader. And officials have also warned that there are some authorities, including some budgeting powers, that don't shift to acting leaders.

"We need these leaders in place to ensure the readiness of our force," said Army Secretary Christine Wormuth, speaking at the ceremony. "And we need to end all of this uncertainty for our military families." She noted that officers and their families are in limbo, waiting to see if they will move or not to new bases, states and jobs.

In a recent comment, Tuberville said it is up to the Democrats to schedule votes on the nominations. "I am not stopping anyone from getting confirmed, and I am not stopping anyone from voting. Democrats could simply put these nominations up for a vote, but they clearly don't want to," he said.

Democrats have said that doing individual votes on each of the nominations would take months, and would eat up valuable time that the Senate needs to address other issues. In the past, senators have routinely voted en masse on large numbers of nominations and doing individual votes more rarely on ones that are less likely to be unanimous.

Smith addressed the issue in a letter to the force on Thursday, laying out the need for the Corps to continue modernization efforts and broader programs to improve warfighting.

"Until the Senate confirms our 39th Commandant, this guidance will serve as our reference point," Smith said. " I cannot predict how long this process may take, but waiting is not an option for Marines, so we will move out as a team – just as we would in combat."

Tuberville has blocked efforts to have Senate votes on all nominations for senior military jobs because he disagrees with the travel pay policy. And congress is now out on summer vacation, which means there will be no action on the jobs for weeks.

Adm. Michael Gilday, chief of naval operations, will step down on Monday. Adm. Lisa M. Franchetti, the current vice chief, has been nominated to take over.

Further complicating things, Air Force Gen. CQ Brown has been nominated to become the next chairman of the Joint Chiefs of Staff, when Army Gen. Mark Milley leaves, as required by law, on Sept. 30. The current vice chairman, Adm. Christopher Grady, would serve as acting chairman.

Brown, whose term as Air Force chief goes for another year, will remain in that post. Gen. David Allvin has been nominated to be the next Air Force chief, if Brown moves to the chairman's job.