I was partially listening to an interview on TV, perhaps of someone high in the chain of command. He said that federal officers are undermanned. That a city could have many thousands of police officers, but that the FBI perhaps has 1200 for the entire country (I'm guessing, can't remember). Point is, federal officers have counted on the cooperation of local police forces in order to do enforcement. Because of democrat mayors forbidding or blocking their police force from cooperating with federal compliance, the feds are severely handicapped in applying force.
So you're right, perhaps the national guard may have to be activated.
Federal officers include much more than just FBI agents. ATF, Border patrol, US Marshalls, Homeland Security, and others have been used in Portland, DC and other places. President Trump just said a day or two ago there are 50,000-75,000 such federal LEO ready to be used if necessary.