I think General Milley is using smoke and mirrors to cover himself. In this letter, Milley reminds those serving of their oath to uphold the Constitution; Milley intentionally omits their oath to support the president. Milley is claiming the president is violating the Constitution and therefore soldiers who uphold the constitution Constitution MUST refuse to be loyal to the president, as if the President's actions are an enemy of the Constitution.
https://pbs.twimg.com/media/EZuLBMvWsAUSKPg?format=jpg&name=large