If a foreign leader issued this threat to massacre civilians with military forces? https://twitter.com/realDonaldTrump/status/1266231100780744704

So I guess we have to talk about 18 U.S. Code § 1385 "Use of Army and Air Force as posse comitatus"

When the National Guard is acting pursuant to the authority of a State's Governor, they are exempt from the Posse Comitatus Act – @GovTimWalz is permitted to use his state's National Guard units for law enforcement.

HOWEVER when the U.S. President federalizes the Guard ( i.e. under 10 U.S.C. § 12406) those Guard elements become subject to the Posse Comitatus Act.

The Posse Comitatus Act, originally signed into law in 1878, is designed to limit the power of Presidents in these cirumstances

So Trump federalizes the NG. What happens then? i.e. what are the circumstances in which the Posse Comitatus Act doesn't apply?

...because the text of the PCA says "except in cases and under circumstances expressly authorized by the Constitution or Act of Congress"

When has Congress expressly authorized a federalized National Guard unit to act in a law enforcement capacity?

The Insurrection Act of 1807.

https://en.wikipedia.org/wiki/Insurrection_Act

This is a good piece on the Insurrection Act - written when Trump was contemplating using the military to round up immigrants. https://www.theatlantic.com/ideas/archive/2019/05/can-trump-use-insurrection-act-stop-immigration/589690/

As a practical matter, in our current system of government, such as it is, Trump can likely deploy federal troops under the Insurrection Act.

You're going to hear a lot of folks saying Trump cannot deploy federal troops because of the Posse Comitatus Act. The truth is, those folks are wrong, the Presidency has too much power, and it is held by one of the worst people alive.