If treason won't stick, "insurrection" would. He defied the chain of command in favor of a foreign power, and stated intent to warn them in the event of conflict.
The weird thing is, Milley's book reads like a confession. He is admitting actual 'treason' in a book he himself (supposedly) wrote. He is proud of his silent internal insurrection against anything Trump. It is the extreme arrogance and delusion of the brainwashed Leftists.
He was so mentally assured of the absolute and fundamental 'EVIL' of anything Trump did or may do, that he was willing to lay the American Military at the use and disposal of the China CCP. And he is proud! To this day he believes that betraying his country in the name of 'get Trump' was the right thing to do. This is the behavior of a crazy person. The guy is delusional.
There is no way to know the depth or the amount, of deeply classified information he sent over to his 'new friends' in Communist Party of China. Anything he told China would have been immediately disseminated to all allies of China around the Globe, including Russia and radical Islamic murder organizations.