The SCUD was first deployed by the Soviets in the mid-1960s. The missile was originally designed to carry a 100-kiloton nuclear warhead or a 2,000 pound conventional warhead, with ranges from 100 to 180 miles. Its principal threat was its potential to hold chemical or biological agents.
It is directly descended from the German V-2. Its warhead is permanently attached to the missile body and thus has a high velocity impact. The first combat use of the SCUD occurred in 1973 in the Arab-Israeli Yom Kippur War. It was later used in the Iran-Iraq war of the 1980s.
"The Iraqis modified SCUDs for greater range, largely by reducing warhead weight, enlarging their fuel tanks and burning all of the fuel during the early phase of flight (rather than continuously). Such a SCUD therefore came down with a relatively heavy warhead and a heavy motor, separated by the light empty fuel tank. It was structurally unstable and often broke up in the upper atmosphere. That further reduced its already poor accuracy, but it also made the missile difficult to intercept, since its flight path was unpredictable.
The Iraqis had four versions: SCUD itself (180-km range), longer-range SCUD (half warhead weight, extra range attained by burning all propellant immediately rather than steadily through the flight of the missile), Al Hussein (650-km, attained by reducing warhead weight to 250 kg and increasing the fuel load by 15 percent), and Al Abbas (800-km, achieved by reducing warhead weight to 125 kg, with 30 percent more fuel). Al Abbas could be fired only from static launchers; all of the others could be fired from mobile or static sites. Only the original SCUD and the minimally modified version were particularly successful."
From: "Desert Victory -- The War for Kuwait" by Norman Friedman, Naval Institute Press, 1991.