What is the Insurrection Act, and why is Trump threatening to invoke it in Minnesota?

President Trump said Thursday he might invoke a centuries-old law known as the Insurrection Act to send troops into Minnesota, threatening a major escalation between the federal government and state officials.

"If the corrupt politicians of Minnesota don't obey the law and stop the professional agitators and insurrectionists from attacking the Patriots of I.C.E., who are only trying to do their job, I will institute the INSURRECTION ACT, which many Presidents have done before me, and quickly put an end to the travesty that is taking place in that once great State," the president wrote on Truth Social...