What is the main resource for war?
It was naturally food and weapons. Food was required for the soldiers to maintain their stamina and energy to live. Weapons were necessary for them to attack their opponents and win the war.
And the Yellow Kingdom didn't have enough of both.
The main producers for food was Duke White's territory. Since their territory focused on farming, they would be able to produce a lot of food as long as enough time was given. Of course, they would need the water resources that were discussed with the other kingdoms.
With Aurora there, she had made sure that the water resource was enough.
But did the White Dukedom's territory had enough food?
The answer is no.