After the Shadows began attacking all the other races openly, there was nothing that any of them could do on their own to stop them. When asked if they would help each other, they didn't comply both out of fear and out of uncertainty of what would happen if they did. However, if Sheridan could prove that his alliance system had the power to face the enemy, they would consider working together against a common foe. Sheridan had only one hope left: the Vorlons. A heated confrontation between himself and Kosh resulted in a Vorlon attack on the Shadows in Brakiri space. They won and with the Vorlon victory came the cooperation that Sheridan needed.
Unfortunately, this victory came with a terrible price: the death of Kosh.