Great Ambition!
Sebastian called for the 3-fold gathering for a reason.
After hearing about the end of the world and the Ancient Cthulu extensively from Eliana, as an intellectual, Sebastian made his calculations and arrived at a basic estimate for when the world would be taken by storm.
According to Eliana, the basic estimate for the destruction of Oceania was in 2 years but there was no specific date or time. Taking this as his cue, Sebastian estimated the time for the start of the world war.
In his calculations, the world war was likely to start when it was only a year to the destruction of the world since by then, the patience of the Empires would have waxed thin and every Empire would be a tense and unsettled state.
When faced with the prospect of extinction, Sebastian was pretty sure that even the exalted Atlantis Empire would not be able to keep their cool.