A Rapid Offensive Unit, or ROU, is a warship type from Iain M. Banks
's science fiction novels about the Culture
. These ships are designed primarily for the war effort during the Culture-Idiran war
. Previously the GCU
, General Contact Unit, had been conscripted into fighting the Idirans
, but was not up to the task of stopping the entire Idiran Armada
So the ROU was created, a dangerous and effective killing machine. The Culture generally frowns on war in general, considering the moral and cultural elevation of war to be high folly. So instead of building beautiful, gleaming warships, the ROU type of ship is usually dark, deadly, powerful and evil looking. The class names for ROUs are along the lines of 'Torturer,' 'Psychopath,' etc. Not at all pleasant and that is the point. (see Culture Ship Names)
The Idirans were a facistic theocracy, as conquerors they picked fights and won many of them, enslaved entire sentient species, destroyed others, and were basically not nice people. The Culture's attempts at changing this, through Contact, and giving amnesty to other races in the Idiran sphere precipitated the conflict. Despite the 851.4 billion war dead, this was considered a relatively small-scale war.
By the time of the book Excession the warships had become much more powerful than their Idiran War counterparts, or had been upgraded as needed. To the point where a single ROU could in a few milliseconds devastate a fleet of Idiran war veteran ships that had been convinced to attack other ships by a traitor.