New Joint Chiefs of Staff Chairman Army General Mark Milley speaks as U.S. President Donald Trump looks on with Vice President Mike Pence and Defense Scretary Mark Esper during a welcome ceremony for Milley at Joint Base Myer-Henderson Hall, Virginia, September 30, 2019.

WASHINGTON — President Donald Trump oversaw the swearing-in of the highest-ranking military officer in the nation on Monday.

Army Gen. Mark Milley was confirmed by the Senate in July to become the next chairman of the Joint Chiefs of Staff. The chairman serves as the principal military advisor to the president.

Outgoing Marine Corps Gen. Joseph Dunford, who took the role of chairman in 2015, swore in Milley during a ceremony at Joint Base Myer-Henderson Hall in Virginia. Trump, Vice President Mike Pence, Secretary of Defense Mark Esper and Secretary of State Mike Pompeo were in attendance.

"We will remain the world's premier fighting force, respected by our friends and feared by our adversaries," Milley said during his remarks, adding thanks to Trump for his leadership.

"I know I have big shoes to fill. I have known Joe Dunford throughout my career, served with him in combat, and I consider him a close personal friend — a friendship forged with the unbreakable bonds of combat that only shared sacrifice can produce," Milley said of his predecessor.

Milley's ascension comes at a particularly tumultuous time as the Trump administration seeks to address Iranian aggression in the Middle East and the denuclearization of the Korean peninsula and to counter emerging competitors such as Russia and China.