The Kree made their first appearance in Fantastic Four #65 (Aug. 1967)
Millennia ago, a race of humanoid reptiles called the Skrulls came upon prehistoric Hala. At that time the Skrulls were a largely peaceful space-faring race, and they set about educating the natives to the point where they could join their trading empire. However, Hala was home to two equally intelligent races, the Kree and the Cotati.
The Skrulls proposed a test to determine the worthier race. This test involved taking 17 members of each race to distant planetoids in different solar systems in another galaxy, providing them with complete supplies to last one year, and then returning at the end of that period to judge the worth of what each group had managed to create. The Skrull starship took the Cotati to a barren moon in one solar system and then brought the Kree to Earth's moon where they created an arena, the Blue Area, and left them there. While the Cotati used their abilities to create a beautiful garden on their formerly-barren moon, the Kree used the rudimentary technology provided by the Skrulls to construct a magnificent city.
A year later the Skrulls returned to judge the accomplishments of the two groups and return them all to Hala. Once back on their homeworld, the Kree leader Morag learned that, although the Skrulls who had retrieved his group had been extremely impressed by the giant city, the other Skrulls were more impressed by the Cotati's success. Enraged, the Kree first wiped out the Cotati and then, when the Skrull delegation protested, killed them as well and seized the Skrulls' starship. They then set about deciphering the technology of the starship.
Within a hundred years of acquiring interstellar technology from the Skrulls, the Kree empire began to spread throughout the Greater Magellanic Cloud. The Kree launched an attack upon the Skrull empire and the peaceful Skrulls were again forced to become war-like to defend themselves. The Kree-Skrull War would continue for hundreds of thousands of years.
At the same time the Cotati on Hala were almost driven to extinction by the Kree. However, a small handful of Kree, the pacifist priests of Pama, hid and kept safe a group of Cotati. Eventually these Kree began worshipping the Cotati that they had kept sheltered. To further hide and keep them safe, the priests relocated the surviving Cotati throughout the universe.
About 250 years later the Kree Science Council, aware that the Skrulls had once created an incredibly powerful weapon called the Cosmic Cube, designed a cybernetic/organic supercomputer called the Supreme Intelligence (sometimes referred to by the honorific "Supremor") to help them in creating a Cube. When the computer became functional, it determined that a Cosmic Cube would be too dangerous to construct and refused the Science Council's request. The computer was then used simply in an advisory capacity.
At the beginning of the Kree-Skrull War, millions of years ago in Earth time, the Kree established a station on the planet Uranus, a strategic position between the Kree and Skrull empires. Through their work at this station, they discovered that sentient life on nearby Earth had had genetic potential invested in it by the alien Celestials. Intrigued, the Kree began to experiment on Earth's then-primitive homo sapiens.
Their goal was apparently twofold — to investigate possible ways of circumventing their own evolutionary stagnation, and to create a powerful mutant race of soldiers for use against the Skrulls. However, although their experiments were successful in creating a strain of humanity with extraordinary and diverse abilities, it was assumed that the Kree apparently abandoned their experiments for reasons which are not yet clear. However it was later revealed that the Supreme Intelligence had in fact predicted its death by the hands of these experiments and ordered their destruction.
Their test subjects, eventually dubbed the Inhumans, went on to form a society of their own, which thrived in seclusion from the rest of humanity. Their city Attilan has frequently been relocated. The Kree abandoned the project eventually, but stationed a giant surveillance robot, Sentry 459, on a small South Pacific island. The Fantastic Four's unwary awakening of the long-dormant Sentry alerted the Kree to Earth's advanced state of evolution. The Sentry's destruction brought the Kree official Ronan the Accuser, with the intent of passing judgment on those who "murdered" the outpost guard. He intended to punish the Fantastic Four, but they defeated him. Ronan's own defeat increased Kree scrutiny of Earth, setting off a chain of events that continues to reverberate.
Over time, and as the war with the Skrull dragged on, the Supreme Intelligence gradually accrued more and more political power and responsibility until finally it was elected as absolute ruler of the Kree. Eventually, the Supreme Intelligence came to be worshipped and an organized religion built up around it.
Not long after the conflicts with the Skrull ended, the Kree again found themselves embroiled in a war, this time with the Shi'ar Empire. This war was much shorter, and was engineered by the Skrulls and the Supreme Intelligence.
The Avengers of Earth became involved in the conflict when the Shi'ar opened a wormhole in Earth's solar system to gain rapid access to Kree territory, unconcerned about the damage that would be caused to Earth's Sun as a result. Their involvement inadvertently set off a chain of events which led to a Nega-Bomb being detonated in Kree space. The bomb explosion, in which negative energy was released into space causing an extremely powerful explosive and radioactive reaction, devastated the Kree empire, and led to its surrender to the Sh'iar. It was eventually revealed that the Supreme Intelligence was ultimately responsible for the bomb's detonation, as the whole thing was a bid to kick-start the Kree race's genetic development.
Kree territory was then annexed by the Sh'iar, with Majestrix Lilandra naming her sister Deathbird as the territories' administrator. Deathbird has since abandoned this post.