*Citations needed.
Erm, I'd argue that personally.
OK lets begin, shall we?
First blight
in -203 Ancient (992 TE), the Grey Wardens gathered an allied army composed of soldiers from the Tevinter Imperium, the tribes of the Ciraine (the lands that would become Orlais), and Rivain to confront the main darkspawn horde, led by the archdemon now identified as Dumat. The Grey Wardens were the leaders of that battle and they rallied all the troops. It was the Grey Wardens who defeated the first blight and saved the whole world.
Second blight
In the following decades, the Blight was again slowly pushed back and the Grey Wardens took command of the war. The archdemon Zazikel was finally confronted and slain by Grey Wardens in 1:95 Divine at Starkhaven in the Free Marches.
Third Blight
The Third Blight began with the awakening of Toth in 3:10 Towers, roughly 200 years after the last Blight. The initial darkspawn attacks occurred in central Thedas in Tevinter and Orlais, but even though the hordes were larger than those previously encountered, a rapidly-organized defense led by the Grey Wardens managed to hold them back. The darkspawn attacks started to focus on the more lightly defended Free Marches while Tevinter and Orlais tried to remain neutral, but pressure from the Grey Wardens eventually brought them into the war. The darkspawn horde was crushed at Hunter Fell in the Free Marches in 3:25 Towers, and Toth was slain by the Grey Wardens. The third Blight remained a relatively short event compared to previous Blights.
Fourth Blight
Not much is told about the Grey Wardens for another 200 years, when Andoral awakened and the Fourth Blight began in 5:12 Exalted. Most of the damage was to the east of traditional Grey Warden territory, in the Free Marches, Antiva, and Rivain, but the Anderfels was also attacked and Hossberg, not far from Weisshaupt, came under siege. This time, both Tevinter and Orlais were lightly attacked and refused to send aid. The hero of the Fourth Blight was an elven Grey Warden named Garahel, who first led the liberation of Hossberg in 5:20 Exalted and then managed to gather an army from the Free Marches to support the Grey Wardens. Garahel's army marched north and faced the main horde in 5:24 Exalted at Ayesleigh, where Garahel perished after personally slaying Andoral.
Too long didn't read: The whole word was saved 4 times solely because of Grey Warden's brilliant tactics and strategy. The Grey Wardens were granted right of conscription for this very reason. They were meant to be above all laws. The chantry itself supported this right during the first blight.