Assuming the story is real and the words he spoke reflect his actual intent, then it is treason. However, unless additional details emerge supporting Milley's actual intent to follow through with his words, I don't see this holding up under legal scrutiny. His words are not necessarily treason if they were intended to trick the other general into complacency, or were empty politeness.
Assuming the story is real and the words he spoke reflect his actual intent, then it is treason. However, unless additional details emerge supporting Milley's actual intent to follow through with his words, I don't see this holding up under legal scrutiny. His words are not necessarily treason if they were intended to trick the other general into complacency, or were empty politeness.