Comment by ethbr1

It's a complicated bit of American constitutional / federal law. Tl;dr...

The US military cannot be used to perform domestic policing functions (Posse Comitatus Act https://en.wikipedia.org/wiki/Posse_Comitatus_Act ), except in times of insurrection or when state unable or unwilling to suppress violence that threatens citizens' constitutional rights (Enforcement Acts https://en.wikipedia.org/wiki/Enforcement_Acts ).

Hence Trump's continual (and false) claims that the cities he's targeting are lawless and dangerous places.

The above applies to federal US military forces. The laws specifically exclude the US Coast Guard. Non-military federal forces (FBI, ICE, etc) are also excluded.

It also, in the more complicated quirk, excludes state military forces (i.e. "National Guard" units). These forces can be activated under a variety of different legal frameworks (see https://www.nationalguard.mil/Portals/31/Resources/Fact%20Sh... ), some of which allow their use for domestic police functions (Title 32 and SAD), because they're still under the command of the state governor (who can use military forces to perform domestic policing functions inside their state or a neighboring state).

There's also a special exclusion for Washington, DC, as technically the president is sort of its governor for many purposes.

Given that background, what actually happened...

- Trump activated National Guard units under Title 10 (aka federal active duty service), because this doesn't require the consent of a state's governor

- Trump then deployed these units to several cities, some with the support of Republican governors and some without the support of Democratic governors

- The administration's legal team realized performing policing functions with the above forces was on extremely shaky ground

- Therefore, they mostly claimed (loudly) that they were deploying "the military", but in actuality used them for extremely limited, non-policing purposes (picking up trash, talking to tourists, guarding federal buildings, guarding other federal agents performing law enforcement functions)

- After state governments sued, the courts generally agreed the deployment was unlawful ( https://www.reuters.com/world/us-supreme-court-rejects-trump... )