The Insurrection Act (10 U.S.C. §§ 331–335) lays out several scenarios where a president may deploy federal troops domestically. The key point is that the law uses very vague standards:

• “Whenever there is an insurrection” in a state and the governor requests help.
• Whenever the president “considers” that unlawful obstructions or rebellion make it “impracticable” to enforce federal law by normal means.
• Whenever people are being denied constitutional rights, and the state cannot or will not protect them Source: ai