In this case, Trump invoked Section 10 U.S.C. 12406, which allows the President to federalize the National Guard if the country is facing an invasion, rebellion, or if the President cannot execute federal laws with regular forces. He argued that the protests in Los Angeles constituted a rebellion.
Comments
"Trump said in his memorandum requesting the National Guard that the protests in Los Angeles "constitute a form of rebellion against the authority of the Government of the United States".
https://www.bbc.com/news/articles/cj93d3r0zz0o.amp
https://www.military.com/benefits/reserve-and-guard-benefits/whats-difference-between-title-10-and-title-32-mobilization-orders.html