At close, the broad OMXS index had lost 0.1 percent after being up 0.9 percent earlier in the afternoon.
Among the biggest losers are technology trading company Addtech, down 5.2 percent, and defense group Saab, down 2.8 percent.
The turnaround was clearly visible after US President Donald Trump threatened Iran that "an entire civilization will die tonight" in a post on Truth Social, which, among other things, caused the price of oil to tick up.
It was also grim on the stock exchanges in the rest of Europe, where London, Paris and Frankfurt all ended in the red while Wall Street opened clearly lower.
During Wednesday night, US President Trump's new deadline for Iran to, among other things, open the Strait of Hormuz, expires; otherwise, new acts of war threaten.
