Eren knew he couldn't involve Feng Yu in this mess. That's why he was in a hurry, ever since the barrier over the Holy City had disappeared.
It was only a matter of time before the other Overlords became involved. Before that happened, he wanted to take care of this matter.
The Holy Priestess was dead and his clone couldn't fight. The forces of the Western Continent were the weakest they had ever been, so there was no point in fighting a large-scale war.
That was especially the case since he wasn't going to gain anything from it.
Still, he declared war for a simple reason. The entire war was nothing but a distraction. It was a false smoke screen for the other Continents keeping an eye on the Western Continent.
The only person that caused him concern was the Dragon Queen, who hadn't woken up yet. He had left a few blood guards behind to guard her room, while leaving a letter for her before leaving.