Footage from a human rights advocacy group appeared to show police firing tear gas at protesters in Tehran. WSJ’s Sune Rasmussen explains why some Iranians took to the streets after the military admitted to shooting down a passenger jet by mistake. Photo: Associated Press Iranian President Hassan Rouhani said European troops stationed in the Middle East could be in danger, suggesting that a collapse of the 2015 nuclear deal would threaten the region’s stability and security. The remarks come a day after Britain, France and Germany took the first steps toward reimposing sanctions on Iran following the sharp increase in tensions after the killing of Iranian Maj. Gen. Qassem Soleimani and Tehran’s attacks on military bases in Iraq that house U.S. troops. …