§ Of the Rings of Power and the Third Age The Silmarillion

When therefore Sauron saw his time he came with great force against the new realm of Gondor, and he took Minas Ithil, and he destroyed the White Tree of Isildur that grew there. But Isildur escaped, and taking with him a seedling of the Tree he went with his wife and his sons by ship down the River, and they sailed from the mouths of Anduin seeking Elendil. Meanwhile Anárion held Osgiliath against the Enemy, and for that time drove him back to the mountains; but Sauron gathered his strength again, and Anárion knew that unless help should come his kingdom would not long stand.