Why side with Sauron?

[Hot, crispy nice hobbit]

You know, the enmity between Gondor and men of the East had been around long before Sauron took refuge in Mordor? The people of Gondor and Arnor (North Kingdom) were descended from the Numenoreans (Men of the West). In their heyday, the Numenoreans wer great Mariners and explored many places in the Middle-Earth. But in the latter days, they became corrupted by their pride and fear of death. Their last king, King Tar-Minastir, landed south of the Bay of Belfalas, a place called Umbar. (Yes, that the place were the Corsairs of Umbar came from) The Numenoreans mixed with the Haradrim people there, and became used to their evil customs. Those Numenoreans were called Black Numenoreans.

Anyway, there had always been bad blood between those the people of the East and those of the West... When the Dark Tower was broken in the Second Age, there had been invasions of Gondor by the Easterlings, Wainriders, and Haradrim even though Sauron had been cast out of Mordor.