The attorney general of Washington, D.C., Brian Schwalb, says he is suing President Trump to stop the “unlawful deployment” of National Guard soldiers to the streets of the nation’s capital. Mr. Schwalb and his attorneys argue that the president has no authority to use the military to conduct everyday police business in the city. 

This is the first instance of Washington leadership legally challenging the National Guard deployment since it began on August 11, though the city did force the Trump administration to back away from a complete takeover of its police department.

Mayor Muriel Bowser publicly thanked Mr. Trump last week for his help in reducing crime in the city. She has signed an order to continue cooperation with the federal government for law enforcement purposes through September 10, at which point Mr. Trump will need a congressional resolution in order to continue deploying soldiers. 

Mr. Schwalb is suing to remove the troops immediately, however. In his lawsuit, which was filed on Thursday, Mr. Schwalb argues that the president’s “unprecedented, unlawful actions have subjected the District to serious and irreparable harm.”

“The deployment of National Guard troops to police District streets without the District’s consent infringes on its sovereignty and right to self-governance. The deployment also risks inflaming tensions and fueling distrust toward local law enforcement. And it inflicts economic injuries, depressing business activities and tourism that form the backbone of the local economy and tax base,” Mr. Schwalb argues. 

The attorney general says that if the takeover of Washington is allowed to continue, the president will take that as a sign that he can take control of any major American city in violation of the Posse Comitatus Act, which restricts the military’s authority to operate within the United States. 

Mr. Trump has been threatening for weeks to send soldiers or large numbers of federal agents into cities like New York and Chicago, and on Wednesday he said that he was likely to take control of New Orleans. 

While Mr. Schwalb concedes in his lawsuit that the president has the authority under federal law to take control of the capital’s National Guard units in order to maintain the peace in emergency situations, he argues that “Congress gave the President essentially no authority to oversee the District or control its governance.” 

“Even when the President assumes or exercises control of the National Guard, Congress has sharply limited his authority to use the National Guard for domestic law enforcement purposes,” he writes.