To the shock of the Iranians, rather than breaking off the offensive, the Iraqis kept up their drive, and a new force attacked the Iranian positions around Basra. Following this, the Iraqis launched a sustained drive to clear the Iranians out of all of southern Iraq. One of the most successful Iraqi tactics was the "one-two punch" attack using chemical weapons. Using artillery, they would saturate the Iranian front line with rapidly dispersing cyanide and nerve gas, while longer-lasting mustard gas was launched via fighter-bombers and rockets against the Iranian rear, creating a "chemical wall" that blocked reinforcement.