In the prelude to the Four Years War, Admiral Kentin led a fleet consisting of over 100 warships, which was first detected by Starfleet after the Arcanis IV massacre of reference stardate 1/9212.21. (FASA RPG modules: The Orions: Book of Common Knowledge, The Orions: Book of Deep Knowledge, The Klingons: Star Fleet Intelligence Manual, The Klingons: Game Operations Manual)
Kentin made a surprise invasion of the Rigel system on stardate 1/9301.04. The Orion Space Navy made no resistance, apart from a few scattered units that attacked and were quickly destroyed. Meanwhile, Federation ships were seized, though there were less of these than expected, perhaps due to Rigellian preparations.
According to Kentin, their aim was not to occupy the Orion Colonies, but to invade the Federation. However, it was over a year before they did so, and the Orions generously entertained the Klingons until they left. Hostilities finally broke out between the Klingon Empire and the Federation on stardate 1/9409.29. (FASA RPG modules: The Orions: Book of Common Knowledge, The Orions: Book of Deep Knowledge)