Souron can hardly be said to have been defeated by the White Council it was ruse on his part to pull their attention there, he simply "fell back" to Morder and almost immediately reoccupied Dol Guldur.
The Fellowship got its *** kicked in Moria.
Boromir got his *** kicked.
Gondor lost the Battle of Osgiliath where Farmir got his *** kicked.
The Battle of the Pelennor Fields was not decisive as it is stated there is still another army unfought near the city.
The Army that marches on Morder more or less is lost too prior the the Deus ex Mechina.
The background battles we are told of are almost all victories for Souron. The one where theodens sun dies, the attack of the orcs that rescued Gollum from the Mirkwood elves, all the battles that destoed Anor and reduced Gondor to its current size. Even victorious battles were accompanied by so much loss that they acomplished little in the long run ( the war of the dwarves and the orcs).
If you go furthur back, Elves and men really didn't win anything but minor skirmishes in the first age. In the Second, Souron surrendered to Numenor and conquired them from within. While he lost the last battle, man and elves hadn't even fully recovered at the end of the third age.
It is also made very clear that the war of the rings was also a Pyrrhic victory - the elves all leave ( arwen dies alone in a deserted Lorian), it is heavily implied that the ents will soon be gone, Dain is dead, Theoden is dead, Denathor is dead, it is implied the Dwarves will soon be gone ( Dain's heir being the last of his line.), etc.
If there is a theme that runs through out it is try as we might and gain a victory here and there, the war is ultimately hopeless. lotr just chooses to focus on the temporary victory.