I had posted this in my Vleast Campaign Page but thought it deserved a post as it is the basics behind the entire campaign. Enjoy.........
Vleast is a planet full of life and rich in minerals. The Imperium wants these minerals so first developed an outpost there in 910.M41. Before long most of the inhabitants had decided to move off-world and go elsewhere for minerals with only a few million staying. A few years later in 975.M41 the Tau expended their borders and began 'diplomatic negotiations' with the now almost tribal population. The humans did not accept the Tau initially but after years of living abandoned by the Imperium they eventually gave in and surrendered to the Greater Good.
The Tau quickly began colonising the world and used their advanced technology to keep the native animals,which plagued the Imperial forces, at bay. However, a Splinter Fleet from Hive Fleet Leviathan had set its eyes on the bio-rich planet also and in 978.M41 the invasion of Splinter Fleet Perses began. The Tau quickly evacuated under the invasion as there was not a sizeable enough military force nearby to effectively defend the planet. Perses was free to feed and rejuvenate itself on the bio-rich world.
The Tau soon realised their mistake, if Perses gained Vleast and was allowed to digest all of its biomass then the Tau Empire were doomed to be devoured by the newly created Hive Fleet. As such time was of the essence and within 6 months the Vleast Command had been created and was en-route to Vleast. This Coalition was mainly comprised of warriors from Fal'shia as it was the closet of the Tau Sept worlds.
When the Tau arrived in the system they found that Imperial forces had already arrived, an unexpected enemy. However, the Imperial forces had not expected the Tyranids and had only arrived to repel the Tau (This was part of the Imperium's plan to stop the 3rd Sphere Expansion). The Imperium forces contained many Imperial Guard veterans skilled at jungle combat and also a company of the Salamanders who were to spearhead the assault. The Imperial Navy had only a few small ships after having destroyed the Tyranid space fleet and so did not attack the much more powerful Tau force. The Tau kindly ignored the Imperial ships and quickly deployed its troops to join the now planet-bound war.
The Imperial forces on Vleast had secured a strong base of operations which the Tyranids sent wave after wave of troops upon to no avail. The Tyranids cared not for the losses as they continued to devour the rest of the planet and gain ever more biomass. The Tau aimed to stop the Tyranids from devouring the planet and began a separate mobile war against the Tyranids leaving the Imperials alone. Much of the jungles had been devoured and only black rock and infested, ruined outposts remained.
The Tau were very successful in their initial attacks as their fast moving forces could easily outmanoeuvre the slow Tyranids who did not have many large creatures due to the speed at which they needed to replenish troops. The Tau used their knowledge gained from Hive Fleet Gorgon to eliminate the few synapse creatures so as to break the synaptic web and hence the Tyranids strength.
However, the Tyranids had learned lessons too and quickly adjusted their plans to fight both enemies. More large creatures were produced to combat each army’s weaknesses. Shrikes fought the Tau with a speed they could only match by fleeing; Tyrannofexes shot down Tau skimmers at range with ease; Tervigons were sent to the Imperial bases to overrun their position with gaunts charged with rage and poison.
But as soon as the Tyranids adapted so did their enemies. Back and forth the war swayed with one side gaining an advantage before losing it to the enemy’s new plans. However, this could not continue indefinitely for the Tau or the Imperium. They both needed new supplies and troops before the Tyranids eventually wore them out. Neither force could kill the Tyranids faster than they produced more. To make matters worse the Tyranids had an endless supply of biomass as when one creature fell, it was reabsorbed and used to create another.
Both sides had to accept this and in 979.M41, after months of bitter fighting they finally allied to destroy the Tyranids. In addition to this, not soon after the fragile alliance had been made, the Eldar of Saim-Hann arrived to provide aid in exterminating the Tyranids. They had predicted that if this Splinter Fleet was allowed to grow that it would eventually destroy dozens of Eldar worlds and several precious Craftworlds.
The Vleast Campaign had begun in full. The Tyranid defeat became a possibility to all the other races, now that they had (temporarily) allied.