Declaring war and following through with it are two different things. While Ashton was hell-bent on fighting the Morgans and making them pay for whatever they had done to countless people, he did not have enough soldiers to face off against more than ten thousand soldiers that the Morgans had under their command.
That was only taking into consideration their own forces. Nevertheless, they were also going to call in favours from other barons which could increase the strength of their forces by up to forty to fifty thousand soldiers.
But that was only a part of his problems. He would need more rations, weapons, armours everything that an army would need to fight for an uncertain amount of time. However, he didn't have any of it.