[Boromir88]

Rhod, and had Saruman won in Rohan half of the forces defending Minas Tirith would have been wiped out. No one would have come to Gondor's aid except perhaps the Dunedain...if they ever met up with Aragorn. Karen Wynn Fonstad estimates the forces during the Siege of Gondor were a little over 11,000 (this includes the Rohirrim). Looking at the numbers we are given in the text, this seems to be a relatively good guess. The Rohirrim set out with 6,000 men, that's over half of Gondor's forces. Gondor would have no chance of winning. There was no hope for a military defeat of Sauron, but the war itself still served a purpose, it kept Sauron thinking that someone in Gondor (that being Isildur's heir) had the Ring and was going to use it against him. Without the victory during the Siege of Gondor and the Battle of Pelennor Fields, which leads to marching an army to the Black Gate, the quest would have indeed failed.