...Which he already did, by sabotaging Kirk's engines so that when Kirk bombarded the Klingon homeworld with long range torpedoes, the Klingons would come and kill Kirk. And when that failed, he had a backup plan that thankfully didn't require a long lead-in time, because the Vengeance
was a very fast ship...
This doesn't exactly tell us how long Marcus had been planning for this, though. All the hardware and personnel might have been at his fingertips already, without any need for him to be part of a secret cabal or anything.