Weeks before Trump ordered additional Federal agents to go to Portland, the city had been under nightly attack by groups of mostly White anarchists, Antifa, BLM extremists, and Fascist-Left radicals. Federal buildings had been defaced and damaged. Federal personnel had been obstructed and injured.
This is not a crowd of entirely peaceful protestors. Many in the mob declare their intent to overthrow the government. They openly promote street violence and riots. They have committed serious Federal crimes, offenses that carry decade-long sentences.
The local authorities have allowed the city to be overrun with a reign of lawlessness. They have not protected Federal facilities or personnel.
So it was necessary and legal for Trump to increase the contingent of Federal personnel and to have them protect Federal buildings and to make arrests. Yes, he should have sent in much larger forces much sooner, well before it got to this point. But at least he is finally getting serious about protecting Federal property and throwing the would-be insurrectionists behind bars for a while.