Thirty years ago, in a future not too dissimilar to ours, the world was sowing the seeds of its own destruction. Designed to bolster manufacturing and create worldwide economic equality, the world designed omnic robots which were built by A.I. centres around the world called Omniums. Eventually however the unthinkable happened, the Omniums went rogue and began churning out military robots to destroy humanity. When governments failed to stop the Omniums the UN created an international task force to fight them. Bringing together the best and brightest from around the world this task force named 'Overwatch' would wage an asymmetrical war against the robots. The small 5-Man strike team was made up by German soldier Reinhardt Wilhelm, Swedish engineer Torbjorn Lindhorn, Egyptian sniper ace Anna Amari and two soldiers from the United States' Soldier Enhancement Program: long-time friends Gabriel Reyes and Jack Morrison. Reyes became the team's leader but it was Morrison who became its figurehead, moulding the team into an effective fighting force. They fought the Omnic robots in what would become known as the Omnic Crisis. It lasted for years and years, displacing India’s population, ravaging Australia, devastating Siberia and besieging the Korean Peninsula with a highly adaptable giant sea-robot. But despite the adversity there were triumphs. The Russians defeated and shut down their Omnium and South Korea created mecha suits to fight the sea-robot. The Crisis claimed many lives on both sides but finally the humans, thanks in no small part to Overwatch, prevailed.
When the crisis was finally over the UN looked to establish world peace. Although around the world Omnics were not considered equal, they allowed a group of rebel omnic monks to remain in the Himalayas and they, including Tekhartha Zenyatta, taught peacefully that robots possessed a soul. The West African city of Numbani became the centre of Human-Omnic cooperation and the Australian government gave the Australian Omnium back to the remaining omnics in hopes of establishing peace. This decision displaced and then radicalised the Outback locals who eventually formed an organisation called the Australian Liberation Front which violently resisted their new omnic neighbours. But most importantly Overwatch became much better funded and diversified into a variety of fields including science, medicine and travel. Morrison was promoted to Strike Commander while Reyes was put in charge of Blackwatch: the covert ops division of the organisation. But the world viewed them as heroes. The youth of that time, the so-called ‘Overwatch generation’, catapulted them into superstardom and Overwatch inspired Amari's daughter Fareeha to join the Egyptian Army and become a hero herself.
As the years went on Overwatch expanded its roster as its resources and scope were increased, transforming from a strike force into a worldwide peacekeeping organisation. Some of the talented individuals they picked up included Dr. Angela Zeigler, a Swiss nano-biologist and medical expert who disagreed with the militaristic stance of Overwatch, Winston, a hyper-intelligent gorilla who escaped an uprising on the Horizon lunar colony, Lena Oxton, a British pilot who suffered a life changing accident in a prototype teleporting plane, Genji Shimada, the heir to a Japanese criminal empire nearly beaten to death by his brother and resurrected as a cyborg, Mei-Ling Zhou, a Chinese climatologist who focused on understanding the environmental and resource based issues of the world and Jesse McCree, a former American criminal who was drafted into Blackwatch after being captured in an Overwatch sting.
Overwatch led the world into a long era of peace but conflict still remained. The Vishkar corporation which had been rebuilding the world after the Omnic Crisis began enforcing strict laws and totalitarian decrees on the people they were helping, decisions that were questioned by some within the corporation including one Satya Vaswani. In a favela in Rio de Janeiro this policy caused a popular uprising led by DJ turned freedom fighter Lucio Correia Dos Santos, who became a celebrity overnight. In Russia the Siberian Omnium reactivated and plunged the country into war again, which made former athlete Aleksandra Zaryanova a national symbol. In Antartica Mei's research was sabotaged by unknown forces and she was forced into cryostasis for years. In Australia the Australian Liberation Front managed to blow up the Australian Omnium causing a nuclear explosion that ruined the Outback. The survivors calling themselves Junkers turned it into a mad, cut-throat society where only the strongest survived. In France Overwatch agent Gerard Lacroix was murdered by his wife Amelie who had been reprogrammed as the Talon sleeper agent and super-assassin Widowmaker. And while the Sea-Robot was kept in check by the South Korean's MEKA program it was forced to draft in pro-gamers including Starcraft world champion Hana 'D.Va' Song in order to continue to fight it off.
But worst of all were the allegations directed at Overwatch itself. Negligence resulting in high-profile mission failures; corruption and mismanagement; weapons proliferation; human rights abuses; and more emerged seemingly out of nowhere. Quickly the heroes of Overwatch became subjects of suspicion and hatred and a UN committee was set up to investigate these allegations. This inquiry would eventually lead to the dismantling of Overwatch but not before one final shocking event.
As the allegations mounted a split began within the organisation centred on Reyes and Morrison. Both never saw eye to eye after Morrison was promoted and now their feud reached breaking point. Morrison advocated for holding Overwatch together while Reyes disagreed. Although we don't know exactly what his agenda was, he is likely linked to attempts by rogue elements in Blackwatch to bring down the organisation. At Overwatch's Swiss HQ the two men fought each other resulting in an explosion that destroyed the base and reportedly killed them both, although their bodies were never found in the wreckage. That act was the death knell for Overwatch and the organisation was disbanded 20 years after the end of the Omnic crisis. Its members took different paths. Many like McCree became mercenaries, a group including Mercy and Reinhardt carried on their heroism without the organisation while others like Winston watched and waited for the days of heroes to return.
With Widowmaker still active, two Junkers Mako Rutledge and Jamison Fawkes going on a worldwide crime spree, the Shimada crime family retaining its power in Japan despite the defection of its remaining heir Hanzo, a gentle but defensive Bastion robot reactivating and wandering the world, Jack Morrison, now a masked vigilante named Soldier: 76, stealing Overwatch technology without caring who gets in his way and Gabriel Reyes now reborn as the terrifying assassin Reaper hunting down and killing Overwatch's former agents one thing is clear:
The world needs heroes.
Edit 06/11/15: With confirmation of Reyes and Morrison as Reaper and S76 some details have been added to them and McCree.
Edit 2: Added details about D.Va, Genji and Mei as well as some extra info to make the story flow.
Edit 3: Changed some errors surrounding D.Va and Winston's lore#
Edit 08/11/15: Added details about Anna Amari, Mei and the state of the world.