The title of this article is conjectural. While the content of this article is based on official information, the actual name of the subject is conjectural, and is yet to or cannot be officially named.
Defense of Minas Anor and Osgiliath
Gondor successfully defends both
Unknown but a formidable force of the dark soldiers of Sauron, and his allies
Unknown but a formidable enough force
Unknown, probably moderate
Unknown, probably heavy
Defense of Minas Anor and Osgiliath was a siege/ battle between the Dúnedain of Gondor and the hosts of Mordor, shortly before the formal formation of the Last Alliance.
loss of Minas Ithil, Anárion was able to assemble a large enough force to successfully prevent Sauron's forces from taking Osgiliath and Minas Anor. The enemy was driven from Ithilien and into the mountains. However, it was only a temporary relief as Sauron's forces would soon regroup to once again become a threat to Gondor, leading Elendil and Gil-galad to form the Last Alliance.  
The Lord of the Rings, Appendix B: The Tale of Years (Chronology of the Westlands), "The Second Age"
The Silmarillion, Of the Rings of Power and the Third Age ↑
The Second Age, "The Last Alliance" The Atlas of Middle-earth,