Not so fast
Armies would fight until %10~%20 losses, then they will flee.
Now, what happens after, depends on the situation, motivation of the enemy and his final goal.
1st Bulgarian empire, emperor Samuil was at continuous war with the Byzantium empire.
In 1014, in battle at Klyuch, the Byzantium emperor Vasilius II captured 15,000 Bulgarian soldiers and nobles.
In a swift maneuver, the hair of the Bulgarian emperor, Gavrail Radomir, decimated small Byzantium army and killed with his spear the Byzantium emperor's favorite general.
In his rage, the Byzantium emperor Vasilius II ordered ALL of the captured 15,000 Bulgarian soldiers to be blinded and left just one eye to every 100 soldiers, to lead them back home.
Thats 13,500 blind soldiers, led by 1,500 soldiers with one eye, trying to find their way home.
The Bulgarian emperor Samuil had a heart attack at the sight of the blinded army and died 2 days later.
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Then we have the infamous king Vlad Tsepesh "Dracula" who applied the scorched earth tactics on the invading Ottoman empire army, supplementing it with impaling thousands of captured Ottoman soldiers and posting them on the side of the road, stretching from the border of his empire, all the way inland, so the invading Ottoman army will be watching their impaled comrades for days.
The plan worked and the Ottomans retreated back, demoralised.
There is a reason for the phrase "I will go medieval on his ass!".