evidence: Mullen told me recently that service in this administration comes with a unique set of hazards, and that Mattis was not unaware of these hazards. "I think back to his ‘Hold the line’ talk, the one that was captured on video," Mullen said, referring to an impromptu 2017 encounter between Mattis and U.S. troops stationed in Jordan that became a YouTube sensation. In the video, Mattis tells the soldiers, "Our country right now, it’s got problems we don’t have in the military. You just hold the line until our country gets back to understanding and respecting each other and showing it." Mullen said: "He obviously found himself in a challenging environment." [...] To some of his friends, though, Mattis was beginning to place his reputation at risk. He had, in the fall of 2018, acquiesced to Trump’s deployment of troops to the U.S.-Mexico border, and he was becoming contemptuous of a Pentagon press corps that was trying to perform its duty in difficult circumstances. [...] The next day, he met Trump in the Oval Office. Mattis made his case for keeping troops in Syria. Trump rejected his arguments. Thirty minutes into the conversation, Mattis told the president, "You’re going to have to get the next secretary of defense to lose to ISIS. I’m not going to do it." He handed Trump his resignation letter, a letter that would soon become one of the most famous documents of the Trump presidency thus far.