U.S. General Mark Milley formally transferred responsibility as Chairman of the Joint Chiefs of Staff to General C.Q. Brown on Sept. 29, the Pentagon announced.

The chairman is the highest-ranking member of the U.S. military. Milley served in the role for four years before retiring on Sept. 29.

In a speech thanking the general for his service, U.S. Defense Secretary Lloyd Austin said Milley "led the world to help Ukraine fight back after Russia's imperial invasion."

Milley also mentioned Ukraine in his farewell remarks.

"We supported Ukraine in their fight for freedom against Putin's war of aggression and strengthened the NATO alliance," he said.

Milley recently spoke with CNN about the Ukrainian counteroffensive, saying there was still time to achieve results and reporting that Russia had amassed over 200,000 troops in Ukraine's occupied territories.

Milley's successor, General C.Q. Brown, was formerly the U.S. Air Force Chief of Staff. He was  tapped for the chairman role by U.S. Presidnet Joe Biden in May.

"We must focus on integrating our military power within our services, across the joint force, and with our allies and partners to deter aggression," Brown said at the transfer ceremony.