Trump now wants to use military force to take California

[Whack-a-Mole]

@Stranger_On_A_Train

The President does, in fact, have the authority to call up the National Guard and other state militia per 10 U.S. Code § 252 - Use of militia and armed forces to enforce Federal authority and 10 U.S. Code § 253 - Interference with State and Federal law. The governor of the pertinent state ostensibly still has control over these forces; however per 10 U.S. Code § 254 - Proclamation to disperse:

How does the Posse Comitatus Act figure in to all of this? (really asking)

The Posse Comitatus Act is a United States federal law (18 U.S.C. § 1385, original at 20 Stat. 152) signed on June 18, 1878, by President Rutherford B. Hayes that limits the powers of the federal government in the use of federal military personnel to enforce domestic policies within the United States. Congress passed the Act as an amendment to an army appropriation bill following the end of Reconstruction and updated it in 1956, 1981 and 2021.