This is about actions taken last summer to bring under federal command units of the California National Guard and to deploy them, primarily in the LA metropolitan area, in response to civil disturbances. It was the first in a series of attempted similar actions including to federalize the Oregon National Guard and deploy it to Portland and to federalize the Illinois National Guard and deploy it to Chicago, which ended when the Supreme Court found the relevant statute, 10 U.S.C. § 12406, did not give the President the power to take such actions.

There's two key arguments. First is that §12406(3) was not met because the president hadn't shown that they were unable to execute the law with the regular military. This follows straightforwardly from Trump v. Illinois :

When granting a stay of the district court’s injunction, we concluded that the President “likely acted within his authority in federalizing the National Guard under 10 U.S.C.…