The planet Hala was the homeworld for the prehistoric, blue-skinned Kree, who shared the planet with a sentient plant race called the Cotati. The Kree race were almost out of the stone age when the Skrulls discovered them. At this time in Skrull history, they were contacting intelligent races and providing them with technology, thereby allowing those uplifted races to participate in a vast trading empire. Finding two sentient races inhabiting Halas, the skrulls decided to hold a contest. They transported groups of Cotati and Kree to the Earth's moon, and bade them create something of worth - when the Skrulls returned, they would award the race they adjudged the victors with technology.
The Kree, led by their cheiftain Morag, built the mysterious Blue City of the Moon, currently used by the Inhumans and Uatu the Watcher as a home. The Cotati built instead a magnificent garden. When the Skrulls returned, they made their judgement - the Cotati were the winners.
The Kree slaughtered both the Skrulls and the Cotati, and seized the Skrull technology. Thus began the extreme hatred between Skrull and Kree. Morag returned to Halas, slaughtered the remaining Cotati that he could find (some would remain hidden), and conquered the rest of Halas, founding the Kree Empire.
The Years Of Empire
Over time the Kree scientists would come to dominate the Kree Empire, building an artificial intelligence which could assimilate the finest minds in the Kree Empire. This construct was known as the Supreme Intelligence, and it ascended to primacy, guiding the Kree Empire with its massive intellect and subtle plots.
The Kree came into contact with many other races, enslaving those who would submit and destroying those who would not, such as the Aakon. The Kree freely plundered technology and resources for their empire without hesitation. Over time many Kree would interbreed with subject races, their offspring being considered to be members of the Kree race. The pureblood blue Kree would become a minority in their own Empire, although the vast majority of the positions of power would remain bastions of Blue Kree exclusivity.
The Kree have been aware of Earth for millenia, and in the dawn of Humanity's pre-history they created an experimental group of genetically-altered humans who would take the name of the Inhumans. Setting their small outposts and weapons caches to the automatic defenses of their Sentry automations, the Kree abandoned Earth for thousands of years, letting their experiment run in hopes of later returning to enslave and use the Inhumans as cannon fodder in their wars of conquest.
The Kree were ade aware of Earth again when the Fantastic Four, while on vacation, accidentally discovered one of their outposts, and defeated its guardian Sentry. They dispatched the head of their judicial department, Ronan the Accuser, and he too was defeated by the Fantastic Four.
After the defeat of their agent, the Kree dispatched a small cruiser to Earth to investigate, commanded by Colonel Yon-Rogg, who dispatched the most decorated hero in the Kree Empire, a pink-skinned Captain Mar-Vell. Mar-Vell would betray his Empire to protect Earth, and later become the Protector of the Universe and fight against Thanos.
The Supreme Intelligence would involve Mar-Vell and the Avengers in their ongoing war with the Skrulls. The Supreme Intelligence was of the idea that the Kree race had reached the end of its possible evolutionary progress, and made several attempts to use Human meta-abilities to jump-start the Kree, to no avail. Finally, having come to the conclusion that the only way to return the Kree to an upward evolutionary spiral was to introduce catastrophic adversity, he manipulated the war known as Galactic Storm so that a nega-bomb would destroy much of the Kree race, in the theory that the survivors would have superior genetic potential and that the adversity would act as a catalyst to further evolution. Thus, the Kree Empire finally fell - by their own hand. Most Kree were not aware that their doom was granted by their own ruler - rather, they blamed Earth and the Avengers.
The Shi'ar Dominion
Empress Lilandria of the Shi'ar appointed her sister Deathbird as the Viceroy of the territory formerly known as the Kree Empire. A Kree representative, the war-hero M-Nell, was admitted to the Imperial Guard, and fought alongside them against yet another plan of the Supreme Intelligence.
Many different splinter groups formed from the ruins of the Kree Empire. Many of the old military, headed by Zey-Rogg (scion of Mar-Vell's old commander Yon-Rogg, who commanded the spy mission to Earth), formed the Kree Peace Battalion, collaborators with with the Shi'ar occupation. The Kree Star Force seemed to have been co-opted by the Shi'ar as well, with Ronan the Accuser serving as appointed by the Shi'ar Imperial court.
Halas would host the Kree Resistance Front, who desired to be free from both the old military heirarchy and the Shi'ar domination. They would attempt to recruit Genis-Vell to their cause.
Several groups of Kree attempted to take revenge on the Earth and the Avengers. They failed. A group of Kree refuges attempted to secure a home among the Inhumans, but they were too aggressive, plotting against the Inhumans while at the same time trying to beg acceptance.
Ronan the Accuser would obtain the Inhumans, and force them into the service of the Shi'ar against various rebel groups.
The Coming of the Ruul
Eventually, during one of the rare meetings of the Galactic Councils, a new race would make themselves known. Called the Ruul, they placed themselves at the service various races and the Galactic Coucil. Meanwhile, the Galactic Council themselves were debating the fate of Earth; many of them had grown to hate the "interference" of the metahumans of Earth, even when that interference was a positive thing. The Galactic Council came to a decision - Earth would become a dumping ground for their own criminal elements, knowing that Earth's forces would be stretched to their utmost to defend themselves.
The Galactic Council accepted the offer of the Ruul to organize and run the isolation of Earth from the rest of the galaxy, and the Shi'ar would appoint Ronan the Accuser the warden of Earth, and the commander of their headquarters there, the Citadel. The Citadel and several watchtower posts in space maintained an interdiction, and the importation and release of alien criminal threats began.
The Ruul took this opportunity to liberate the Kree Supreme Intelligence from SHIELD custody, revealing to some of the heroes of Earth that the Ruul were actually the Kree in diguise. They revealed the secret Kree plot: that the Supreme Intelligence had used the Forever Crystal to grant some of the Kree the ability to evolve themselves into efficient forms for whatever task in front of them - savage, swift, ultrastrong forms for combat, heavy durable forms for work, or cerebral, clever forms for intrigue or diplomacy. They planned to convert Earth to a living planet, and harness that power for its own ends - conquering the galaxy. Secret Kree fleets launched into war against all of the races of the Galactic Council, a war which is ongoing today.
Racial Characteristics
The Kree are utwardly humanoid to a large degree, Kree bodies are adapted to environmental characteristics on Hala that are un-Earthlike: notably, higher gravity and a higher nitrogen content in the atmosphere. Although the Kree cannot breathe Earth's atmosphere without special apparatus, their denser bodies afford them about twice the average human being's strength and endurance. Despite their physical superiority and relatively advanced technology, the Kree race has reached the pinnacle of their evolutionary development, having not significantly changed in any way in tens of thousands of years.
The Ruul
Some of the blue Kree have been remade by the Forever Crystal into the Ruul, who have the ability to adopt through forced evolution forms which are optimal for the task at hand. They can shift from form to form at will, usually to warrior, worker, or intellectual forms as in the illustration. It is unknown if there is a process by which Kree of mixed heritage or blue-skinned "purebloods" can be raised to Ruul form from the old race. The approximate numbers of Ruul versus old Kree which are still in existance are unknown. The reaction of the majority of the Shi'ar-dominated Kree is unknown, nor how many worlds of the old Kree Empire the Ruul have regained.