The Kree originated on the planet Hala in the Milky Way Galaxy.
Kree closely resemble humans except for their blue skin. They are also physically stronger than humans, posssessing twice the endurance, strength and stamina of the average human. The Kree are an evolutionary stagnant race due to a member of their race attempting to gain control of a cosmic artifact by the name of "The Crystal of Ultimate Vision". As punishment, the Crystal froze the evolution of the Kree in place. In order to remedy this they attempted to mate with other races which produced pink-skinned Kree who are similar to caucasian humans.
The Kree rule an interstellar empire spanning many worlds. A defining trait of their society is their long lasting war with the Skrulls, a rival alien race.
The Kree are ruled under a monarchy, with an emperor.
The Kree evolved on the planet of Hala in the Magellenic Clouds. They initially shared the planet with another race called the Coati. Milennia ago Hala was visited by an alien race called the Skrulls who came upon prehistoric Hala. The Skrulls were a peaceful spacefaring race who set about educating the natives in order to include their world in the Skrullian tradiing empire. However since Hala was home to two races, the Skrulls decided to hold a test in order to determine which race was worthy of receiving their aid. A band of Kree and a band of Coati were taken to a Skrull testing area on the moon of Earth, a planet in the Milkyway Galaxy. The Kree created a magnificent city as their entry while the Coati created a beautiful garden. The Skrulls were more impressed with the Coati and decided to support them. Enraged, the Kree band killed their Skrullian judges and Coati rivals and took control of the Skrull's starship. They piloted the ship back to Hala and within a hundred years they had acquired technology rivaling that of the Skrulls and began to spread throughout the Greater Magellanic Cloud. They launched an attack on the Skrull Empire which was to be the start of a Kree- Skrull war that would last for hundreds of thousands of years.
When the Kree learned that the Skrulls had created an extremely powerful weapon called the "Cosmic Cube" the Kree created a cybernetic/organic supercomputer called the "Supreme Intelligence" to help them create a cube of their own. The Supreme Intelligence was the databank of the greatest minds of the Kree and it determined that creating a Cosmic Cube would be too dangerous. The computer was intead used in an advisory capacity since then. As the war with the Skrulls dragged on the Supreme Intelligence acquired more and more political power and responsibility until it was finally elected the supreme leader of the Kree.
In order to circumvent their own evolutionary stagnation and create a powerful army of soldiers, the Kree conducted experiments on several alien races. These experiments were determined failures however and all but five of the races they had created were destroyed. Chief among the five were the Inhumans who had been created from manipulation of the early humans of Earth. The Inhumans went on to form a society and culture of their own which thrived for many years separate from humanity. The Kree also left a Sentry on Earth to keep an eye on their experiments.
Many years later, a human exploration and adventure team by the name of "The Fantastic Four" accidentally activated the Sentry which alerted the Kree to Earth's advanced state. A Kree official by the name of "Ronan the Accuser" was sent to Earth in order to pass judgement on those who had "murdered" their Sentry. Ronan was defeated however which lead to even greater interest in Earth. The Kree, now under the control of Ronan launched an invasion fleet towards Earth in order to prevent the Skrulls who had also sent an invasion fleet from capturing it. The conflict was ended however when the Supreme Intelligence managed to create a distraction in the form of a young human within whom he unlocked his entire genetic potential. The human managed to stop the fleets singehandedly before dying of the strain while the Supreme Intelligence - who knew that war with the Skrulls was pointless - regained control of its people.
After the Skrull Empire was decimated by an attack by Galactus - a cosmic force of nature who's purpose is to maintain the cosmic balance by consuming planets - conflict between the Kree and the Skrull lessened. The Kree would be confronted with another threat however in the form of the Annihilation Wave - a gigantic army from the Negative Zone, a Universe outside our own - which launched an invasion of our Universe and targeted the Kree Empire as one of its first targets. The Kree were initially overwhelmed but together with various other forces who were assembled to fight the Annihilation Wave, the Kree managed to prevail. In the course of battle, the Annihilation Wave manage to permanently cripple and disable the Supreme Intelligence. Ronan kills it out of mercy and is made supreme leader of the Kree.
Their troubles were not over however. Shortly after the Annihilation Wave, the Kree Empire was assimilated by the Phalanx, a techno-organic race which sought dominion of the Universe. The Phalanx isolate Kree Space from the rest of the Universe but they are saved by heroes from a group called "Guardians of the Galaxy" and Nova, a member of the inter-galactic police organisation named Nova Corps.
The Kree would once again become entangled with the Inhumans. During the Skrull invasion of Earth, the Inhumans who had now made their home on the Moon contacted the Kree for aid. The Kree lead by Ronan agreed and after the Skrulls were defeated, the Inhumans - now vowing to never allow themselves to be victims again - traveled to Hala in order to claim their "rightful" place as rulers of the Kree. Ronan admitted to not being comfortable as leader and agreed to let the Inhuman king Blackbolt become ruler of the Kree.
As part of the agreement, the Inhumans had promised to jump-start the Kree's stagnant evolution and Ronan was promised the hand of the Inhuman princess Crystal Amaquelin. Trouble would come once again however, this time from another rival interstellar empire; the Shi'ar. The Shi'ar had recently come under the rule of Vulcan; who was the son of two human slaves in the Shi'ar court. Vulcan was a mutant which granted him extraordinary powers on par with those of the Inhuman king Blackbolt. Vulcan had been raised in slavery on the Shi'ar throneworld and had returned to the Empire to claim revenge. Due to his royal blood however (anyone born in the royal palace of the Shi'ar like Vulcan was is considered of royal blood), he had been convinced by Deathbird - sister of the then current Majestrix of the Shi'ar; Lilandra - to seize the throne for himself and become Majestor.
Vulcan conducted expansionist politics in an attempt to restore the Shi'ar to their former glory. They attacked and conquered several Kree border worlds which placed them in direct conflict with the Kree. The Kree offered asylum to the former Majestrix of the Shi'ar; Lilandra and her friends in the Starjammers (a band of space pirates wanted by the Shi'ar) who's captain was Vulcan's human brother Alex Summers. The Shi'ar Imperial Guard attacked the wedding of Crystal and Ronan which promted the Kree to launch complete war on the Shi'ar.
After many battles, the Inhuman's planned to detonate a bomb which would not only defeat the Shi'ar but turn everyone in the galaxy into Inhumans. Vulcan learned of this plot which was to be powered by Blackbolt's voice (a mere whisper of his was enough to shatter mountains) and attacked Blackbolt. The two fought vicously but Blackbolt - having regrets about the nature of the bomb - had disabled the Terrigen Mist agent which was to convert the galaxy to Inhumans and overpowered the bomb when Vulcan made him so angry that he screamed. The resulting explosion tore a hole in space-time and killed both Blackbolt and Vulcan.
With their Majestor dead and military devastated, the Shi'ar surrendered to the Kree. Gladiator - former leader of the Shi'ar Imperial Guard - assumed the mantle of Majestor although at this point the Shi'ar were little more than a Kree puppet state.