The generals are afraid that if troops are called on to contain, repel, or shoot rioters, the military will have taken sides in a domestic race conflict. This happened in the 1960s when troops were deployed against race riots in the inner cities. This set off years of racial conflict in the ranks, with sabotage, insubordination, and attacks on officers in the field in Vietnam. It took a generation to restore order, morale, and unit effectiveness in the US military.
Why would Trump put up with his instructions to the military being disobeyed?
What other choice does President Trump have without the support of his own party?
The military brass has 3 1/2 track record of non support of POTUS. The generals don’t want to be seen publicly supporting President Trump. Even worse they have aided and abetted the ongoing coup against POTUS. Witness the still excellent military career prospects of coupist Vindmand and the Corona boat captain. The military is not with the POTUS and the citizens. That much should be clear by now.
For example, in the 1980s, Reagan's immediate national security staff contrived the rickety Iran-Contra arrangement in order to get around the resistance of the US military brass to supporting the Contras in Nicaragua because doing so might lead to a repeat of the Vietnam debacle. Or, as a naval officer friend of mine personally familiar with the episode put it, "The brass was ready to fight a nuclear war with the Soviets but having to put helicopters over the jungle chasing guerillas gave them nightmares."