Yan could maybe have rejected the ceasefire using his authority as the second imperial prince. It was a hefty position, even if the imperial court didn't give him the respect his siblings got.
But he wasn't a military man or otherwise a part of the army. They were also in a smaller city on the countryside in the East. The imperial family's influence wasn't as great as it was in the capital.
He would not have many allies or much trust left if he forced the military officers of Rittel to listen to him and refuse the ceasefire. Since the majority clearly wanted the ceasefire, it was an uphill battle to get them to agree to refusing it, and it wasn't like Yan would get much from it anyway.
"Don't let the troops see that you're disappointed about the ceasefire," Garad commented when he saw Yan's frown. He didn't bother keeping up the pretense of formality anymore. What was Yan going to do, kill the old man for being rude to a member of the Imperial Family?