Within three hours, Microsoft threw itself into the middle of a ground war in Europe — from 5,500 miles away. The threat center, north of Seattle, had been on high alert, and it quickly picked apart the malware, named it “FoxBlade” and notified Ukraine’s top cyberdefense authority. Within three hours, Microsoft’s virus detection systems had been updated to block the code, which erases — “wipes” — data on computers in a network.

Then Tom Burt, the senior Microsoft executive who oversees the company’s effort to counter major cyberattacks, contacted Anne Neuberger, the White House’s deputy national security adviser for cyber- and emerging technologies. Ms. Neuberger asked if Microsoft would consider sharing details of the code with the Baltics, Poland and other European nations, out of fear that the malware would spread beyond Ukraine’s borders, crippling the military alliance or hitting West European banks.

Before midnight in Washington, Ms. Neuberger had made introductions — and Microsoft had begun playing the role that Ford Motor Company did in World War II, when the company converted automobile production lines to make Sherman tanks.