I. Army A is attacked by (or sometimes attacks) Army B, a larger force or at least one with an obvious advantage. Army A is soon routed and flees the field in full retreat. Army B pursues them straight into an ambush at which Army A’s much larger reserve force (usually divided into left and right wings for a surround) overwhelms Army B, the initial attack being a ruse.
II. Army A plans an attack on the much larger Army B. To do this they have an alliance with Army A-2 (or sometimes A-2 is a division of Army A). At the moment of battle Army A-2 withdraws, leaving Army A to attack alone where they are overwhelmed. (Sometimes this is because A-2 has a secret alliance with Army B, sometimes it’s because their leader just happens to hate the leader of Army A, and usually a collusion of some sort twixt A-2 and B.)
Are their technical terms for these other than ‘ambush’ or ‘surround’ for I or ‘last minute treachery’ for II?
I. In terms of cavalry this sometimes called the caracole technique. It was used by mongols and other mounted archers. Though the caracole is slightly more involved, and involves charging at the enemy before wheeling around at the last minute and appearing to retreat, all the while firing arrows.
II. Army A-2 is the definition of “turncoat”
II was recently done on an ep. of Burn Notice. The guy they (Michael & Co.) wanted to take down got convinced that Sam was on his side, along with a bunch of mercenaries Michael signed up. They went to confront the bad guy’s rival (also an evil d-bag), and Sam and his band of hired goons all silently withdrew, leaving the two bad guys to shoot it out (fatally IIRC for both of them).
It’s happened in history as well. One example is Philip II Augustus of France, who promised all manner of knights and siege equipment to Richard Lionheart when he went to war with his father and then withdrew at the last moment, which made Richard have to flee before the battle even started. The crazy thing is that he and Richard somehow trusted each other enough to team up again later.
I don’t know that that was preplanned, though. Another example was that of Bosworth Field, where two of Richard III’s commanders betrayed him, the Duke of Northumberland refusing to engage Henry VII, Lord Stanley waiting until Richard III was actively engaged with Henry VII and then attacking Richard’s troops from behind.
Before the battle started, Richard III had taken the Stanley’s son, the Lord Strange, hostage to guarantee Stanley’s loyalty, and when the battle started, threatened to execute him if Stanley didn’t move to engage Henry’s forces. Stanley just sent back a note saying, “I have other sons.”, which is kind of chilling, all things considered.
Respectfully, the caracole is not a feint. Its purpose is to allow all the individuals in the unit to discharge ranged weapons (arrows or pistols) with the front ranks firing and then wheeling towards the back to reload, resulting in a more or less continuous stream of fire.
It is a standalone tactic that can be combined with feints, but is not a feint in and of itself.
But that never works out well in the long run for A-2, because a few survivors of A will always assemble a plucky group of mages, templars, and rogues to avenge the treachery. (Often with a caracole.)
There are plenty of examples of it happening mid battle. One of the more famous ones was during the rise of Cyrus the Great. Cyrus was leading a revolt against the Median King Astyages, during a battle outside Median city of Pasargadae the Median general Harpagus switched sides mid-battle, resulting in the capture of Astyages and Cyrus’ victory. According to legend (well Herodotus actually) this was due to Astyages earlier forcing Harpagus to eat his own child in revenge for not killing Cyrus as a child as he was ordered to do (some might say it was a little short sited for Astyages allow Harpagus to lead his army after this event).