The feudal system of Bretonnia requires all would-be Knights of the Realm to first prove their mettle as Knights Errant by demonstrating their bravery and prowess against a worthy foe. Knights Errant wander the Old World seeking advesaries to test themselves against, righting wrongs and slaying wicked beasts in the name of the Lady of the Lake. When an Errantry War is declared, however, these young knights will gather from all across Bretonnia to face a foe declared worthy by the King himself!
There have only been a few Errantry Wars throughout Bretonnian history. The first was declared in the Bretonnian year 471(IC 1449) during the Arabyan invasion of Estalia. Fighting alongside the Imperial Knightly Orders, the Knights Errant and their allies drove the forces of the Sultan Jaffar from the Old World. where they decided to follow him south and break his power once and for all in a crusade. Word of the victory came just as a fresh army was about to set sail, the large army of knights errant now having nowhere to go decided to attempt clearing the Border Princes of Greenskins. Since then, relations with the Dwarves have been good.
The second Errantry War was called in 1223(IC 2201) in an attempt to rid Bretonnia of Orcs, who had been a source of trouble since the founding. While that was nigh-impossible the young knights attempted, and many mountainous areas was added to the dukedoms that used to be dominated by Orcs.
King Charlen declared the third in 1442(IC 2420) this time to rid the entire world of all Greenskins. While it clearly was an impossible task, the knights still left Bretonnia to fight the everything from Black Orcs to Gnoblars in such great numbers that the kingdom itself was left vulnerable against Greenskin raiders. In 1510(IC 2488) a massive army disappeared while crossing Death Pass, an area controlled by the Black Orc warlord Morglum Necksnapper. this ended the Errantry War.
The fourth Errantry War was declared by king Louen Leoncoeur during the Storm of Chaos, in response to Emperor Karl Franz plea of aid against Archaons Chaos Incursion. This fourth Errantry War is not canon in the Age of Reckoning timeline associated with WAR.