On a breezy day in September, an ominous text message appeared on Fredrik Ekstrom’s phone: Go immediately to the war room.

Mr. Ekstrom is chairman of Nasdaq Clearing, an outpost of the American stock exchange company in Stockholm, which processes futures trading and acts, in part, as a shield against contagious losses in the global financial system. He headed toward an ordinary conference room outfitted with extra phones, video hookups, data terminals and policy manuals, designed to be a command center in case of a financial crisis. Until then, the war room had been used only for drills.

Other senior executives were waiting when he arrived. They described an unfolding disaster. A trader named Einar Aas was having a spectacularly bad day.

Mr. Ekstrom knew the name. Mr. Aas worked from his seaside home in a remote Norwegian fishing village, trading futures contracts tied to the price of Scandinavian electricity. Known for taking huge risks, Mr. Aas had once been one of Norway’s richest people.