Former Defense Secretary James Mattis said he had “no choice but to leave” the Trump administration in an interview with The Atlantic, but was reluctant to share additional details about his acrimonious departure.

Mattis pointed to his resignation letter, which made clear he had differences with President Trump, in explaining his decision to leave after nearly two years as Defense secretary.

The general said it would not be appropriate for him to speak more loudly about any differences he might have had with Trump.

“If you leave an administration, you owe some silence,” he said, telling The Atlantic that his decision follows the French concept of “devoir de réserve,” or “duty of silence.”

Read more at The Hill.