In Imperial Year 1185, the Great Bridge of Myrddin became a key objective in the conflict between Fódlan's three countries. Whichever side held it could freely invade either the Empire or the Alliance.
Upon Byleth's return to the Black Eagle Strike Force, Hubert suggests to invade the Alliance capital of Derdriu in order to remove the tense three-way standoff between the Empire, Holy Kingdom of Faerghus and Alliance, due to Claude's potential threat to the Empire. Before they can reach Derdriu, they must cross the Great Bridge of Myrddin.
Claude, having anticipated a potential Empire attack, had stationed several Alliance soldiers (and some of his former classmates if they had not been recruited during the academy phase), led by Judith. He warned Judith to fall back if the enemy was too strong, but Judith secretly decided to disobey this order and fight to the bitter end.
When the Black Eagle Strike force arrived, Judith and her garrison were ready to greet the invaders. Judith also had incoming allied reinforcements in the form of Acheron and his troops, hoping that he would arrive in time. Upon Judith's death, Acheron immediately turned tail and fled with his men, unwilling to die for someone he loathed. This secured the bridge for the Empire, who could now freely invade the Alliance.
Following Byleth's return to the Golden Deers, Claude decided to attack the Great Bridge of Myrddin in order to remove Edelgard's threat to Fódlan. The bridge had been secured by the Imperial Army, led by Ladislava. Ladislava hoped to pin her enemies down in the form of allied reinforcements in the form of Acheron and his troops, although they were quickly taken down by the Alliance.
Ferdinand also made an appearance here if he was not recruited during the academy phase. The battle ended once the enemy commanders were taken down, giving the Alliance a vital opening into Empire-held territories.