Defense Secretary James Mattis arrived at the Pentagon Saturday for his first day of work, after being sworn in the day before.

The retired four-star Marine general arrived promptly at noon. He was greeted by Chairman of the Joint Chiefs of Staff Marine Gen. Joseph Dunford, who had previously worked for Mattis at various times in his career.

#Mattis arrives at the Pentagon for his first day. Greeted by @GenDunford and the press. pic.twitter.com/8AtTHUyLLp — Kristina Wong (@kristina_wong) January 21, 2017

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Dunford saluted Mattis as he arrived, and could be heard addressing him as "Mr. Secretary." Mattis did not return the salute, instead shaking the chairman's hand before he led in walking up the steps into the building.

As some observed online, not returning Dunford's salute showed his adjustment to his new role of civilian leader of the Pentagon, despite more than 40 decades in the military.

Notice how Secretary #Mattis didn't return Gen. Dunford's salute. He knows his role as SECDEF. This is good. https://t.co/mi3MJg0cHZ — Bae's Berserkir (@barefootboomer) January 21, 2017

Indeed, Congress had to pass a waiver for Mattis to exempt him from a law requiring Defense secretaries to be out of uniform for at least seven years. Mattis, who retired in 2013, has only been out of uniform for three years.

Mattis was the first of President Donald Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE's cabinet members to be confirmed by the Senate and was sworn in on Friday, along with Retired Marine Gen. John Kelly as secretary of Homeland Security.

Both nominees were confirmed on Friday with overwhelming bipartisan support in the Senate.