The Boron Campaign was the first large-scale war that involved all members of the Commonwealth. It was fought between the Paranid/Split alliance against the Boron/Argon alliance a few decades after the end of the Xenon Conflict.
The Boron Kingdom had grown continually since discovering the secrets of space travel and the gate network, and as of the year 255 had expanded through a huge tract of space to the east of their home Sector of Kingdom End. Unfortunately for the Boron, the Split had occupied this region of space since discovering space travel themselves 150 years earlier, and due to their warlike nature soon found glee in attacking the new far-flung Boron colonies.
There was little that the Boron could do to stop the increasing and frenzied Split onslaught as it overcame the Boron fleet. However, in the intervening years Split ships had also had skirmishes with Argon deep space mining and exploration vessels and forged an alliance with the Paranid. This move seemed to make them stronger, but instead gained them a powerful new adversary in the Argon Federation which, following their victory in the Xenon Conflict, had been undergoing a cold war with the Paranid Empire.
The Argon Federation desperately needed a new ally, and found it in the Boron in 323. In the year 324, a combined Split/Paranid fleet attacked another Argon deep-space mining fleet. The Argon Federation responded by formally declaring war on the Split and Paranid, officially beginning the Boron Campaign.
The Split had continued to push back the Boron until the year 326, when a combined Argon/Boron fleet defeated the Split/Paranid assault fleet attacking the Boron homeworld of Kingdom End. The Argon and Boron fleets continued to push back the Split and Paranid fleets until the latter were both forced to concede defeat in the year 350. The defeat was not taken amicably, and the Split and Paranid fought a short war between themselves which ended in 352.
The Boron Kingdom never regained all of its former territories, some of which ultimately ended up under Teladi control, and by the beginning of the Second Xenon Conflict they were still the smallest and weakest of the factions in the X-Universe. However, they did forge a powerful and long-lasting friendship with the Argon Federation and formed the Foundation Guild, which would continue to dominate X-Universe politics for centuries to come.