Loghain (and the narrative) push that Cailan was inept in his handling of the Fifth Blight, but his admiration of the Grey Wardens and his quick response to the threat doubtless played a role in the Fifth Blight being the shortest in history. Much of Ferelden still reveres the Wardens, no doubt thanks to the King's propaganda, despite their previous exile and Loghain's rebranding of them as traitors. This combined with the blow the army deals to the horde at Ostagar allows the HOF the time and ability to gather the necessary allies for a second assault at Denerim.