Washington (CNN) Former Secretary of Defense James Mattis in an interview released Thursday defended his resignation from the Trump administration last year as well as his decision not to outwardly criticize the sitting president, even in his new book.

"I had no choice but to leave," he told The Atlantic in the interview. "That's why (my resignation) letter is in the book. I want people to understand why I couldn't stay."

Mattis resigned as President Donald Trump's first defense secretary in late December after the President announced plans to withdraw US troops from Syria, citing irreconcilable policy differences in a letter to Trump that took many in Washington by surprise. His latest interview follows a string of public statements Mattis has made implicitly criticizing his former commander in chief, with whom he sharply disagreed on matters of international engagement and alliances.

"I've been informed by four decades of experience, and I just couldn't connect the dots anymore," he added.

In this most recent interview, Mattis declined to directly address the character of Trump -- with whom he had an openly rocky relationship marked by presidential Twitter insults -- citing a "duty of silence" he owed the administration.