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There are plenty of people on this board with far more intimate knowledge of 40k fluff than I, and I was hoping to get some help from them. I want to organize a campaign in secret for my friends, and I want to have a very compelling back story to motivate the players. The armies I need involved are:

Imperial Guard
Grey Knights
Craftworld Eldar
The Tau Empire
Chaos

Basically I need a reason for all these different factions to be present on a single planet, although I suppose a system of planets would do just as well. Any ideas about forging an actual story would also be greatly appreciated to get my creative juices flowing. Thanks guys.

in terms of just story ideas, maybe the tau are expanding and find some artifact on the planet. They don't know what it is but accidentally activate it (they just don't know). There's an eldar craftworld in the area and they somehow sense it and decide to go destroy it. At the same time chaos senses it and decide to come to claim it to try and gain power. The grey knights are in the area and notice the chaos incursion and decide to destroy them. They realize when they get there and find the 3 armies fighting they need reinforcements and call for the imperial guard and from there the world turns into a mess.
Hope this helps a bit

The planet is an Imperial hive world near the Tau border. While they still pay their Imperial tithes, the Water caste have persuaded the governor to engage in various other commerce with the Tau. They're doing fairly well at balancing between the two governments until a Chaos warband arrives. They are just raiding aimlessly, but the Eldar (whose Craftworld is relatively close but well-hidden) know of a powerful artifact on the planet and land troops to try to protect it. The Chaos warlord figures out the reason for the Eldar attack, and begins a purposeful search for the artifact. The governor sends an appeal for help to the Imperium, not knowing that his Water caste "advisor" has also called home for military support. A Guard task force is mobilized, as is a Hunter cadre. Before either arrive, the Chaos forces seize the artifact and use it to open a rift, unleashing daemons upon the planet (remember that you need daemons, not just Chaos marines/cultists, to draw the Grey Knights). An Inquisitor of the Ordo Malleus is stationed in the sector and calls in the Grey Knights as soon as he receives word that daemons are abroad. They are the last force to arrive in the warzone. The point I'm seeing to begin the game is that the Guard have fortified one/several of the major hives, the Tau have fortified another/several others, the Chaos forces are expanding in a hurricane of destruction outward from the artifact site (probably in a remote wasteland), the Eldar are distributed in small, autonomous groups pursuing their own objectives (mostly attacking Chaos, but with several other non-obvious goals that bring them into conflict with the others), and the Grey Knights are preparing to make planetfall.

The planet itself is rich in raw materials like adamantine and other very rare and precious resources. An Imperial Mining colony has been mining the resources for the past 250 years. The miners dug too deep and found something from before the great crusade. An ancient machine that has the power to destabilize the planets core and cause a chain reaction to occur that would not only destroy the planet but tear a whole in the very fabric of reality and create a miniature eye of terror. Originally it was meant as a gateway home for the lost explorers from ancient terra who were cut off by the warp storms but the scientists who created it went insane before being killed by what may have been Eldar weapons before the machine could be activated.
After its discovery a techpriest was able to restore power to the site and power up the ancient machine. No one has heard from the colonists since they reported finding the artifact and all shipments from the planet have stopped. Forces from the Imperial Guard have been dispatched to the planet help secure its tithe and investigate the problem. A Tau recon team senses no life on the planet but plenty of valuable ore and raw materials so a force is sent to investigate and secure the planet. When the machine was activated a ripple of evil psychic energy was released which alerted a nearby craftworld as well as the Grey Knights and the forces of Chaos. The Ripple was a plea from the ancient and malignant force that is the Machine, it’s calling for help, its begging to be activated…