But the Mattis critique is foundational: The president he serves, he suggests in his letter, does not understand the value of allies, or the immorality of disparaging and abandoning them. Trump, as my colleague David Frum notes, is abandoning America’s Kurdish allies in Syria to extremism and terror, and he is abandoning Afghanistan to the Taliban. Mattis always knew that Trump lacked an understanding of why autocracy is the enemy of the American idea. But Mattis operated under the illusion that he could change Trump’s views, or at least some of his foolish ways. Yesterday marked the end of the illusion.

Mattis’s departure also means that the United States is entering the third phase of Trump’s foreign policy. In the first year of his presidency, Trump paid attention mainly to domestic issues, and did not afflict America’s diplomatic and national-security establishment with an undue number of his ignorant and damaging foreign-policy views. In the second year, he became more destructively engaged, but he listened, on occasion, to those in his administration who possessed actual expertise in foreign policy. We are now entering the third year of his presidency, and third phase of his foreign policy: Trump alone, besieged, but believing, perhaps more than ever, in the inerrancy of his beliefs.