The League of Assassins operate in secret throughout every country of the world. Called into being centuries ago by Ra's al Ghul, their original mission was to act as his elite army and shield their master from attacks as "The Fang which protects his Head".
Through the years, the League sought out the best killers in all manner of disciplines, from martial artists to snipers. Every member added to the league gave up their life, sworn to the Demon's Head. Only assassins who have already completed a previous hit can be considered for League membership, and its roster is kept strictly confidential from public eyes.
While initially run by Ra's himself, in time, Ra's turned management of the league over to a man from Hong Kong known as the Sensei, who took to calling the league his Society of Assassins. At a later date, a wandering spirit known as Jonah discovered the Society of Assassins and realized that they could be used to destroy the deity Rama Kushna and her mystic land of Nanda Parbat. Inhabiting the Sensei's body, Jonah directed the League to develop sophisticated weaponry. One of Sensei's prized assassins, Hook (Roy Matson), was manipulated by Rama Kushna into assassinating aerialist Boston "Deadman" Brand, creating an agent that the deity could use to oppose Jonah and protect Nanda Parbat.
Eventually the Sensei and Ra's al Ghul had a falling-out and the Sensei left, taking a splinter cell with him, while Ra's turned command over to Dr. Ebenezer Darkk. Under his guidance, the league undertook a series of executions aimed at shipping magnates who were smuggling weapons to South American rebels. When the string of murder reached Gotham City, the Batman became involved. Having fallen from grace after many failures against the Batman, Darkk took Talia Head, Ra's daughter, hostage but was eventually killed when she shot him; causing him to wander into the path of an oncoming train.
This incident saw Ra's' consideration of the Batman as a suitable heir to Ra's operation; future head of the League and spouse to Talia. Though their interactions saw to the conception of a child, Damian Wayne, Batman rejected both Ra's and Talia due to philosophical differences.
Ra's and the Sensei eventually engaged in a struggle for control of the League, a globe-spanning struggle that drew the attention of the Batman when the Sensei attempted to kill the delegates of an international peace conference held in Gotham City. The event saw the deaths of both opponants but they were both resserected shortly there-after due to Lazarus Pits.
The League continued to operate but, under Ra's guidance, remained underground, taking assignments through middlemen and avoiding direct opposition with law enforcement wherever possible. Concerned about future attacks, Ra's assigned David Cain the task of creating the perfect bodyguard. This led to the birth of Cassandra Cain and, as a collateral event, the rise of Lady Shiva.
When Nyssa Raatko killed her father, leadership of the League fell to her. She recruited Lady Shiva to act as the league's new "Sensei", training a new generation of killers until her plans and the new League were disrupted through an intervention by Batgirl (Cassandra Cain).
By the time after the Infinite Crisis the League had been split into three feuding factions; lead by the Sensei, Talia Head and Cassandra Cain (who had fallen under the sway of David Cain and Slade Wilson).
Cassandra's faction saw action against Supergirl (Kara Zor-el), Robin (Timothy Drake) and the assassination of Nyssa Raatko but was left leaderless when Cassandra regained her senses. Stepping into the void were league devotees who hired ex-League member Merlyn to assist them in abducting a young girl called "Sin", who was being trained to be the next Lady Shiva.
During the events that surrounded Ra's al Ghul's return to the living, the Sensei attempted to disrupt Ra's' resurrection, leading to a battle between the two factions and the Batman family in Nanda Parbat that saw the death of the Sensei.
After obtaining Kirk Langstrom's Man-Bat formula, Talia's faction possessed a sectional army of Man-Bat Commandos and continue to be an independant faction residing on Gibraltar even after the return of her father Ra's al Ghul.