Iran's next major offensive, led by then Colonel Ali Sayad Shirazi , was Operation Fath-ol-Mobeen (Undeniable Victory). On 22 March 1982, Iran launched an attack which took the Iraqi forces by surprise: using Chinook helicopters, they landed behind Iraqi lines, silenced their artillery, and captured an Iraqi headquarters. The Iranian Basij then launched "human wave" attacks, consisting of 1,000 fighters per wave. Though they took heavy losses, they eventually broke through Iraqi lines. Operation Undeniable Victory was an Iranian victory; Iraqi forces were driven away from Shush, Dezful and Ahvaz. The Iranian armed forces destroyed 320–400 Iraqi tanks and armored vehicles in a costly success.
Saddam responded by launching chemical attacks against the Iranian positions along the highway and by initiating the second "war of the cities", with an air and missile campaign against twenty to thirty Iranian population centers, including Tehran.