Erm...It took Nazi Germany like...6 years to turn from a heavily restricted (due to Treaty of Versailles) and impoverished country with a small and specifically defensive force to a full-blown superpower with state-of-the-art military tech that could take on the world.
It was 15 years after the Rhineland-Kusari war of late 800 A.S, in 816 A.S. that Kusari and Bretonia went to war.
15 years is a long time, particularly for a house that has bucketloads of resources, wealth and shipyards.