Department of Defence chief of staff Kevin Sweeney has resigned, a month after the Defence Secretary James Mattis announced his departure.
He is now the third senior Pentagon official to announce his resignation after President Donald Trump said US forces would leave Syria.
Rear Adm Sweeney held his post for two years from January 2017, and called it "an honour to serve" alongside his colleagues in the department.
His announcement comes days after General Mattis quit his post early, after initially planning to stay in his role until February.
The former defence secretary hinted at policy differences with President Trump in his resignation letter.
Addressed to Mr Trump directly, the letter described the general's views on "treating allies with respect" and using "all the tools of American power to provide for the common defence".
"Because you have the right to have a secretary of defence whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position," he wrote.
Department spokeswoman Dana White also left her post after the president's surprise announcement on Syria.
In a terse resignation letter, Kevin Sweeney said it had been an honour to serve, but made no mention of Donald Trump.
His departure adds to a sense of uncertainty surrounding the Trump administration's defence and foreign policies since the surprise announcement of the planned withdrawal from Syria.
On Tuesday, Secretary of State Mike Pompeo will embark on a week-long tour of the Middle East designed to reassure allies in the region.