Rutile Quartz no Taikan

Slaine is a humble, provincial commoner in the small Kingdom of Hasenvalia... Or so he thinks until the day the entire royal family is wiped out in a palace fire and Slaine comes to learn he's really the king's illegitimate son. It now falls on him to succeed the throne. Despite his lack of knowledge and experience, Slaine's natural wits and dedication—plus the support of his aide-de-camp, Monica—allow him to excel against all odds. The future for him and the kingdom looks bright—right up until the eve of his coronation, when the great Empire of Galed declares war against Hasenvalia. Faced with such a frighteningly powerful foe, Slaine is forced to make a choice between fleeing into political exile or resisting invasion to the bitter end. Caught between the conflicting agendas of neighboring nations, this boy from a backwater town must learn how to grow into his crown. Witness the birth of a king!

(Source: J-Novel Club)