What the letter said was something that Regan and Rex had not expected but they couldn't accept it so easily.
After knowing Arthur's capability of controlling others' minds, they felt the need to once go to Royal Palace and see on their own to find out about the situation.
Hence, Rex decided to go to Royal Palace.
Still, Derrick was quite faster than their expectation again. The Royal Army was sent to their Palace by the time Rex was ready to leave on his horse for the Royal Palace.
Rex left while Regan stayed back to manage everything.
As he passed the city, Rex could see how the situation was changing so fast. It had already been announced in the whole city that the war was about to begin again.
People were starting to donate to the soldiers on the border whatever they could.
It made Rex sigh in frustration as he wondered how his father could trust Arthur so easily.