Why did operation torch happen?

Operation Torch was attempted mainly because Joseph Stalin was demanding that the allies open up another front against the Nazis. Unfortunately, D-Day was still a year and a half away, so they decided to ramp up the pressure on their only existing front in Europe - North Africa. And yet, although they were making sizable advances there, the entire campaign had been see-sawing back and forth for years, so Operation Torch was a sort of insurance to make sure they would succeed in repelling the Italians and Nazis out of North Africa.