The retired four-star Marine Corps general said in an interview with CBS that aired Sunday that he had intended to serve the full four years, but resigned when he determined his views were not “aligned” with President Donald Trump’s.

Mattis said he disagreed with Trump’s decision on Syria because “we need to maintain enough influence there that we don’t see the same thing that happened when we withdrew from Iraq.” He told CBS pulling out of Syria would undermine the US campaign against ISIS and would betray allies fighting alongside Americans.

“This is how I saw the strength of America — that we keep our alliances together, and keep them tight,” he said. Mattis’ stressing of the importance of maintaining alliances echoed the sentiments he expressed in his resignation letter. He also addresses such policy matters as they relate to Trump in his forthcoming book, “Call Sign Chaos: Learning to Lead.”