British and Australian forces have crossed the Jordan and established themselves in the town of Salt, preparatory to an attack on Amman. But the Turks are somehow wise to their plans, counter-attacking the Allies in great numbers. The British have been hanging but are in increasing danger of being cut-off and forced to surrender. Realising there is no longer any prospect of taking Amman, Allenby gives his men permission to withdraw. They slip back across the Jordan, abandoning Salt to the Turks.
The balance of forces in Palestine and Transjordan now seems to be moving in favour of the Turks. Allenby has to send his best troops to France, where they are needed to meet the German spring offensives. The failure at Salt has discredited the British, as has the publication by Soviet Russia of the Sykes-Picot agreement to partition the Middle East. With the Turks now in the ascendant they see a surge in enlistment from the Arabs of Transjordan as local notables send men to join the winners’ army.
Retreating across the Jordan (Wikipedia: Second Transjordan attack on Shunet Nimrin and Es Salt)