They basically just went through three days of verbal fencing over who did wrong, and Zenjirou was, during that time, just one more young person around the table. Christiano is prideful and, furthermore, he must be feeling like he's the second most important person in that formal company representing the strength of their nation (armed forces), while visiting the neighbouring country, which is also a rival country. Zenjirou didn't really ever emphasize his own status and was just placing arguments on the table, like any other present there. Zenjirou was also cracking relaxed jokes, which seemed to serve Christiano, though obviously they didn't. I'd say, considering Christiano's young age and lack of experience, in combination with Zenjirou lacking any and all kingly bearing, it was realistic enough Christiano made such a critical mistake in the heat of the moment.