The primary function of any government is to provide safety of life, property, and freedom of movement. In a federal system such as ours, that responsibility first falls on local and state government.When they fail or out right refuse to control the situation, the federal government is the backstop. A very simple concept to understand.
Then there is the concept of overwhelming force to prevent future threats to public safety. I have observed first hand in Memphis and DC in 1968 and Columbus 1970 that any law enforcement presence that is not overwhelming to begin with is destined to allow the situation to escalate and become out of control. The presence or promise to use overwhelming force is effective, and its effectiveness is based on numbers and assets deployed, not on skulls bashed.
Finally, America is about civil discourse to decide issues. Violence to resolve issues by force is unacceptable and violence when it occurs must be stopped. We may disagree with Trump on style and substance, but he is correct in sending the message that overwhelming force will be forthcoming from the federal level if state and local government fail in their duty to their citizens.