Chicago has become the ultimate proving ground for President Trump's domestic military experiment, pitting armed federal forces against a Democratic-led city determined to resist them.

Why it matters: The arrival of the National Guard — coupled with Trump's calls to arrest Chicago Mayor Brandon Johnson and Illinois Gov. JB Pritzker — could open a volatile new chapter in American history. • Trump officials say the deployment, which relies on 200 National Guard troops from Texas, is meant to protect federal agents conducting immigration operations. Illinois has sued, calling it an illegal occupation. • The troops aren't supposed to engage in actual policing so as not to violate the Posse Comitatus Act of 1878, which generally forbids the military from civilian law enforcement activities

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