Early on Wednesday, James Mattis spent the last few hours of his time as secretary of defense reminding the employees of the Pentagon and the armed services to “keep faith in our country and hold fast, alongside our allies, aligned against our foes.”

Mattis, who resigned over a disagreement with President Donald Trump’s decision to withdraw all American troops currently serving in Syria and Afghanistan, was one of the last remaining members of Trump’s inaugural cabinet and was often viewed as a stable hand behind Trump’s sometimes surprising and erratic foreign policy.

Read more: AG Healey: Mattis' Retirement 'Troubling And Serious'

To replace him, Trump has nominated Deputy Defense Secretary Patrick Shanahan, a former Boeing executive who has no military experience, to oversee the nation’s military. According to national security expert Juliette Kayyem, while Shanahan may boast strong business credentials, his lack of military experience will ultimately harm the Department of Defense.

“[Shanahan] just talks in these cliches rather than what is the nature of his job, which is to avoid military conflict,” Kayyem told Boston Public Radio on Wednesday. “The military aspect of what it means to be secretary of defense will be lost in the Pentagon.”

Despite the abruptness of Mattis’ resignation, Kayyem said she’s not sure it portends as drastic a change in policy as the president thinks. She pointed out that Trump has reversed his position before, and wouldn’t be surprised if the president ultimately decided to keep American forces in Syria.

“None of [Trump’s comments] are real," Kayyem said. "The troops are in Syria. The wall is not built. The president will say the opposite, and it’s just up to good journalists, and hopefully now, a much more aggressive check [from Congress].”