Sauron retreats - In his desperate bid to capture the Elven Rings, Sauron over-extended himself.  That, combined with the forces from Númenor proved disastrous for the Dark Lord.

"Sauron was driven away south-east after great slaughter at Sarn Ford (the crossing of the Baranduin); and though strengthened by his force at Tharbad he suddenly found a host of the Númenóreans again in his rear, for Ciryatur (Númenórean admiral) had put a strong force ashore at Gwathló (Greyflood), 'where there was a small Númenórean harbour.'  In the Battle of the Gwathló Sauron was routed utterly and he himself narrowly escaped.  His small remaining force was assailed in the east of Calenardhon, and he with no more than a bodyguard fled to the region afterwards called Dagorlad (Battle Plain), whence broken and humiliated he returned to Mordor, and vowed vengeance upon Númenor." ~Unfinished Tales (The History of Galadriel and Celeborn)