The call that came into state capitals stunned governors and their National Guard commanders: The Pentagon wanted thousands of citizen soldiers airlifted to the nation’s capital immediately to help control crowds outside the White House in the wake of the death of George Floyd…

President Trump was drawing instead on an obscure law, changed after the attacks of Sept. 11, 2001, that made it easier for governors to voluntarily send guardsmen across state lines for counterterrorism missions. His action was not an order but a request, essentially inviting states to augment the D.C. National Guard, which he controls, in a potential clash with civilian protesters.

The request had the effect of cleaving state militias along partisan lines, according to interviews and internal Guard documents. While red states jumped to answer the president’s call, governors and Guard commanders in blue states were incredulous. The result was a deployment to the nation’s capital that military historians say appears to have been without precedent: Over 98 percent of the 3,800 troops that arrived in the District came from states with Republican governors.