Because of the thousands of summer tourists, it’s hard to look around Nesebar. But a stroll along its meandering streets and among its half-ruined churches is always worthwhile. In the 6th Century BC, the Greek city-states of Megara and Miletus contended to control of the Black Sea coast of Thrace, which supplied the ancient world with the timber, hides, copper, slaves, and salt its economy needed. Miletus was in the superior position. It managed to establish a colony first, Apollonia (present-day Sozopol), on the southern coast of the Bay of Burgas and to gain control over Anchialo (present-day Pomorie). It was only a century later that Megara found a suitable…