The Battle of Serenity Valley was a battle fought during the Unification War in Serenity Valley, located on Hera, and was considered to be the final decisive battle in the war. The battle began in May 2511, and while not the final battle of the war, was generally considered the end of the war for the Independents, who had lost almost all of the sixteen brigades and twenty air-tank squads they committed to the battle.
The battle lasted seven weeks, with dug-in Independent forces matched up against the superior numbers and technology of the Alliance, before Independent High Command officially surrendered to the Union of Allied Planets. Several commanding officers were killed in combat leaving Malcolm Reynolds to lead 2000 troops. In the last week While The Alliance and Independent High Council were discussing peace talks 400 troops went down to 150. Most decimated were the 57th Overlanders which only had 2 survivors from the original platoon. The Alliance were victorious due to their previously mentioned numbers and technology, as well as a brilliant deep-flank strategy by General Richard Wilkins.
Malcolm Reynolds, of the 57th Overlanders, was a sergeant at the beginning of the battle, but by the end so many Independent officers had died that he ended up commanding over 2,000 troops. He led a spirited defense, relying on digging in and ambushing the Alliance forces to hold them back, often placing himself in danger to do so.
Zoë Alleyne, also of the 57th Overlanders, was Malcolm's Corporal and friend. She also helped in the organization of the leaderless Independent forces, usually accompanying the Sergeant on his various suicidal missions.
Malcolm and Zoë were the only survivors of their original unit, and two of only 150 survivors of the 2000 men that Malcolm had been leading, most of whom died of injuries or starvation during the week-long period while the surrender was organised. The 68% total soldier casualty rate was mainly attributed to this lack of medical support after the battle.
Mal named his ship, a Firefly-class transport ship of which Zoë was second in command, Serenity after this particular battle; the rubric "no one leaves Serenity, you just learn to live there" being a bittersweet reference less to the ship's status as a home as to the catastrophic body count at the battle.
The Battle of Serenity Valley would be the equivalent of the Battle of Gettysburg on Earth during the American Civil War, in that it marked the turning point in hostilities that allowed one side to gain clear momentum that the opposition would be unable to regain subsequently. The Battle of Shiloh was another battle fought during the American Civil War that ended in a Union tactical victory. The term Shiloh ironically may translate to "place of peace," similar to Serenity.
Behind the scenes
A deleted scene available on the Firefly DVD set indicates that casualties from the battle totalled at least 500,000, with some 330,000 being Browncoats.