The enemy at the battle of Jarrab were the Shammar who fought under the banner of the Al Saud's traditional enemy, the Al Rashid.
The battle was joined at Jarrab. It did not go well for Ibn Saud's men. The Ajman contingent of Ibn Saud's forces failed to hold against the Shammar horsemen who broke through and rode towards the gun emplacements. It was clearly time to retreat.