system
Given the following post, respond as if you're commenting on Reddit:
human
[Marvel Comics] What are the Kree and the Skrull fighting over?
gpt
This happened long ago, but the Skrulls have a very old, very powerful empire, with particular traditions. One of these was that, when they came across one world with two intelligent species on it, they would set some sort of contest to determine which species would represent their world on their Throneworld.

Back before the Skrulls became a military power, they had a power based on merchant-trade and exploration. They came across the planet Hala, home to the Kree and also the peaceful Cotati.

They gave 17 Coatati and 17 Kree isolated patches of prepared land in remote star-systems, a year's supplies, and their contest would be to build the superior work in that place, in a year. The winner would have the official 'seat' as an ambassador on Throneworld, permanently.

The Kree had a patch of Earth's moon, the "Blue Area" (where there was a constructed atmosphere). They built a city. The Cotati built a beautiful botanical park, in an identically-barren part of another system. 

The park was, by most standards, the superior achievement (just getting it to *grow* that fast, alone!), and the Kree knew it. Before the judging could happen, the Kree attacked and destroyed the Cotati 'entry'. 

When the Skrull judges arrived and balked at what happened, they attacked and killed them, too. 

They kept the Skrull delegations' ships and technology, adapted them and (by the time the Throneworld found out what happened, over such large distances) had become a vicious interstellar power. 

(In a way not-dissimilar from the origins of the "Evil Federation" on your *Enterprise* television program)

The Skrulls, accustomed to simply taking on the forms and natures of the people they encountered, to blend in, took on the form and nature of a vicious, agressive warlike people, just to survive.

The war had gone on for several thousand years before either side even heard of Earth.

This might be hard for you to understand, but there is no *causus belli* to resolve at this point. There is no issue of land or resources, economics or politics, government or religion involved- the war exists because, to both sides, the war has *always* existed.

 Kree society had reorganized itself to expand into what it saw as a hostile universe, and the Skrulls had redesigned *themselves* as warriors in response. The war continues out of inertia and vendettas, and neither society is able to let go of any of it, for fear of the other wiping them out.

More has happened, of course, but that's the cause.