Former Defense Secretary James Mattis, who served under President Donald Trump for two years, suggested he opposes Trump’s widely condemned decision to abandon U.S-allied Kurdish forces in northern Syria.

Mattis, who resigned in January after Trump announced he planned to abruptly withdraw U.S. forces from Syria, has been reluctant to forcefully speak out against the president, saying military officials shouldn’t be political.

But asked about Trump and his apparent decision to greenlight Turkey’s military assault on the Kurds, Mattis said his resignation says it all.

“I have my private, my personal concerns,” Mattis said during a taped interview with NBC’s “Meet The Press” that aired Sunday. “I keep those private. ... The defense of the country is non-partisan so we have to stay out of those sorts of discussions.”

He then pointed to his resignation letter, in which he urged Trump to maintain strong allies and show them respect, to understand his current stance.

“Only in Washington, D.C., could a public resignation over a matter of policy, a matter of principle, with a page-and-a-half letter explaining why be considered careful,” Mattis told host Chuck Todd.

“It talks about our security being tied inextricably to our alliances,” he said of his resignation letter, which prompted panic from Democrats and some Republicans when it was made public. “I don’t know what more I could say about how I think we ought to treat allies and how I think we should treat those who are adversaries.”