The play starts in Venice and moves to Cyprus when the Turks invade.
Early modern (c. 1500-1750) Venice is a prosperous Italian city and a symbol of law and civilization. It's also full of white people, which makes Othello, a black Moor, stand out among the Venetians. (Check out our discussion of the theme of "Race" if you want to know about the implications of this.) Venice also happens to be renowned for its courtesans (prostitutes). When the English thought about Venice, they often imagined it to be a city full of promiscuous women. Now that's quite a coincidence, given that Othello's plot hinges on Othello's suspicions about his wife's fidelity, don't you think? Check out what Thomas Cory at has to say in his account of his travels to Venice:
[t]he name of a Courtesan of Venice is vamoosed over all Christendom […] The woman that professeth this trade is called in the Italian tongue Cotezana, which word is derived from the Italian word cortesia that signifieth courtesie. Because these kinds of women are said to receive courtesies of their favorites […] As for the number of these Venetian courtesans it is very great. For it is thought there are of them in the whole city and other adjacent places, as Murano, Malamocco, etc. at the least twenty thousand, whereof many are esteemed so loose that they are said to open their quivers to every arrow, a most ungodly thing without doubt that there should be tolleration of such licentious wantons in so glorious, so potent, so renowned a cit." (Coryat's Crudities, 1611)
Eventually, action moves to a military encampment in Cyprus, an island sacred to Venus, the goddess of love. On the island of love, away from civilization and rationality, all hell breaks loose and Iago is able to convince Othello that Desdemona has been cheating on him. At this military camp, Desdemona has lost any kind of support system she may have had in her hometown of Venice, so she's vulnerable to the kind of violence associated with the world of men and military.