The protests had turned into riots.

Two days after a police officer murdered George Floyd by kneeling on his neck, the mayor of Minneapolis called the governor, Tim Walz, asking for help. The police had lost control of the situation. The city needed the Minnesota National Guard.

That same Wednesday evening, May 27, 2020, the city’s police chief, Medaria Arradondo, sent an email to the state commissioner of public safety conveying a similar message, asking the National Guard for “immediate assistance with significant civil unrest.” He said 600 troops were needed.

It was not until that Thursday afternoon that Mr. Walz, a Democrat, signed an executive order activating the Guard. That night, before large numbers of troops were deployed, rioters set a police station on fire. And it took until Saturday, five days after Mr. Floyd had died and three days after Mayor Jacob Frey of Minneapolis had requested National Guard assistance, for the situation to begin to calm.

Ever since, the governor’s response to those requests has been under scrutiny, even more so now that Vice President Kamala Harris has selected Mr. Walz as her running mate.

Mr. Walz, a National Guard veteran, was in charge when his state became the center of a searing discussion about racism and police violence, and as it experienced some of the country’s worst unrest in a generation.