Defence Secretary James Mattis' imminent departure is not good news for America's allies. And at home, it is the first major public rift between the defence establishment and the mercurial President.

The former marine general's resignation letter, in which he said the President should find a secretary of defence more aligned with him, was a "stunning rebuke of the President's leadership, his world view… and his actions that have caused key allies to call into question US resolve and commitments to uphold our alliance obligations", Dr Zachary Abuza, professor at the National War College in Washington, told The Straits Times.