The fact a Secretary of Defense (second in command of the US military) and a Chairman of the Joint Chiefs of Staff (highest ranking officer and principle military advisor) have both said he is unfit and doesn't care about the law.
Jim Mattis, a Secretary of Defense, in a letter titled "I cannot remain silent":
“Donald Trump is the first president in my lifetime who does not try to unite the American people—does not even pretend to try. Instead, he tries to divide us... We are witnessing the consequences of three years of this deliberate effort. We are witnessing the consequences of three years without mature leadership... We must reject and hold accountable those in office who would make a mockery of our Constitution.” [1]
Mark Milley, a Chairman of the Joint Chiefs of Staff, spent his final days in his position making sure that the military understood that they took an oath to the constitution before president. Mark Milley in his retirement speech said:
The fact that they are not complying with the law.