I'm interested in better understanding medieval military tactics. I know that flanking maneuvers are often considered devastating in battles but I can find any good examples of this tactic being decisive or even important in a medieval battle. Can anyone point me to a good instance?
In the late Medieval (1356) period The Battle of Poitiers during the hundred years war is an example of effective flanking.
Mid battle the French lead another charge and the English led a simultaneous charge accompanied by a flanking manuvoure led by de Buch. The French were
"Stunned by the attack, the impetus carried the English and Gascon forces right into the French line. Simultaneously, de Buch's mobile reserve of mounted troops fell upon the French left flank and rear. Fearful of encirclement, the cohesion of the French army disintegrated as many soldiers attempted to flee the field."
The Battle of Issus It is pre-medieval, But the tactics and technology would have been similar. Fought between the Greeks led by Alexander the Great, and the Persians lead by Darius III.
During the battle Alexander with his cavalry was able to break through Darius's left flank and them come at the center of the Persian battle line from behind. Alexander himself charge Darius, who fled the battle field. Naturally his troops did like wise.
See here for more details about the battle.
See here for a list of historical battles(and descriptions) that involved flanking maneuvers.
Offhand, I can think of the Battle of Tagliacozzoa, 23 August 1268. The Guelf and Ghibelline forces were separated by a bridge over a river. Some of the Ghibellines were sent to a ford a few miles away and crossed to attack the Guelfs from the rear. The Guelfs were defeated by the double attack in the first phase of the battle.
If eastern Roman of "Byzantine" battle tactics are "medieval" enough for you, there were "Byzantine" military textbooks such as the Strategikon attributed to Maurice (reigned 582-602). No doubt those textbooks would mention use of flanking attacks and no doubt "Byzantine" generals often used flanking attacks.
Battle of Mohi (1241), during mongol invasion to Europe, where hungarians offered battle to the mongols. Part of the mongol forces under Batu command attacked over the river, while another force under Subutai crossed the river in another side.
Hungarians started to win the battle against Batu, but once Subutai forces arrived from the side, panic in hungarian army forcing them to retreat to the camp. Mongols left a narrow gap to let the enemy flee, so they would be easily killed during the escape.
After the hungarian retreat to the camp, mongols did not feel strong to continue the battle. This is indicative of the effect of the flanking attack over superior forces.