Once, there was peace among all the races; Shinigami, Arrancar, Bounto, Quincy, Superhumans, etc.
The Gotei 13, home of the shinigami, after tough negotiations was finally able to make peace with the quincy. The captain commander at the time was somewhat of a pacifist. He did not believe in war and dreamed for the day where no one would ever have raise their weapons for any reason again. His dream slowly came to life with the start of the the quincy alliance. Now that they were allied, they began to fight side-by-side as the quincy race began to breed and grow into a force that could easily rival any race. Because the quincy originally lived in the in the dimension of the living, Soul Society was soon able to gain access to the quincys' connections in the real world. Because of this, the shinigami were once again able dig up the roots that connected them to the bounto.
The bounto always resented the shinigami as they were looked down upon as just failed experiments. The future bounts of this generation have carried on their ancestry hatred for the shinigami and, the quincy for helping to exterminate their ancestors. The leader of the bounto could find no reason to ally with them. Trying to make amends, the captain commander and the current leader of the quincy at the time promised them an alliance with another race as a sort of gift to prove themselves worthy of their alliance. With the reluctant agreement of the bounto leader, both the shinigami and the quincy tried to come up with a way to get the bounto and super humans to join forces. At the time, the super humans weren't grouped together but, rather a "Every man for themselves" bunch that had to be found.
It took many years to help the superhumans, not only create an organized structure amongst themselves but, gather them all and find a way to recruit others to their ranks. Like an incubator, Soul society helped make the Superhumans what they are today. Now risen to greatness because of the shinigami and the quincy, they wanted to repay the debt in any way possible and as such, allied themselves with the bounto. Now a growing force and realizing that both the quincy and shinigami have honored their word to make amends, they finally joined forces. Soul society quickly became a powerful force.
After a few more years of refining their ties, the captain commander of soul society came to an agreement with the other race leaders that it was time to ally themselves with the arrancar. With only the leader of each race crossing the border into the dimension that is Hueco Mundo, they quickly found the leader of the espada who sat on his throne in Las Noches. The espada stood to his feet as he walked towards the race leaders. He was not as easy to convince to join the alliance with peace and as such was threatned by the captain commander. He explained to the espada that they now had every race on their side and could easily topple the arrancar on his command. Realizing that they were outmatched, the Primera conceded and submitted to the alliance. The captain commander's dream was realized. There was peace, no more reason to fight. It was a happy time as everyone became like buddies working together for a common cause.
Hundreds of years passed and the captain commander has just recently passed away. A new captain commander, one of his descendants, have taken his place. Unfortunately, it wasn't long after the new captain commander stepped up that the arrancar started a revolt. Never truly wanting to be allied with the shinigami, they've been secretly gathering their forces. They attempted to kill the captain commander but to no avail. Because of Soul Society's massive force of races, the arrancar did not stand a chance and as such returned to Hueco Mundo. Truly wanting to make peace with the arrancar by any means nescessary, the current captain commander allowed the ultimate taboo of allowing shinigami to obtain hollow powers. The experiments began soon after the announcement but many died as not all of their bodies could handle the power of the hollow. The captain commander knew that the arrancar knew that Shinigami would never allow such a thing and thought that if the arrancar witnessed this, they would know how serious the captain commander was about making peace with them.
The experiments continued, many of which resulted in powerful hollow that rampaged through the seretei. The current quincy leader, descendant of the now deceased leader, was already growing angry of the fact that the shinigami would ever want to make peace with their greatest enemy. Seeing as now Soul Society seemed to be no better than Hueco Mundo, they left the alliance as they could never make peace with the hollow such as them. The superhumans left soon after as they felt that they no longer needed to ally themselves with the shinigami. The chaos is what pushed them away. The Bounto, feeling that with these vizard experiments they were repeating history, grew angry believing that the shinigami were not truly sorry for the suffering they've been through and as such, sabotaged the experiments. The captain commander was assured that the experiments would be fine but a bounto, the current bounto leader to be precise, messed up their methods which led to the rampaging hollow all over Soul Society. The Captain Commander was blamed for this tragedy and as such, many of the vizard joined with the other races but few are still completely loyal. Now with everything thrown off balance, a great war has begun. In the end will there be peace? Or will there be total annihilation?
The Bleach Universe, lost in turmoil and corruption. Aliiances have been made and broken. This is a war of wars for the Bleach universe.