In chapter 61
of Shadow of the Moon,
the Royal En stresses that is forbidden to invade another kingdom, "even to to suppress an internal rebellion" (for example, General John J. Pershing's incursion into Mexico in 1915 in pursuit of Pancho Villa, who had been conducting cross-border raids into New Mexico). Consequently, unlike in the anime, in the book the Royal Kou himself never pursues Youko into En, but only uses his youma agents. So in this and other accounts of the downfall of Kou, the king is recorded to simply have died after the kirin succumbs to the shitsudou,
the penalty for rejecting the Mandate of Heaven.