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An east-west street of more than 30 miles divides Tehran, Iran’s megapolis of a capital, into two halves: a modern north and a traditional south. Thirty years ago the thoroughfare was named after Reza Shah, the founder of the Pahlavis, the last dynasty of monarchs in Iran. Today it is called Enghelab (Revolution) Street after the turmoil that led to the creation of the first theocracy in the country’s history.

In one of those ironies of which Iranian history is full, on February 11, the anniversary of the Khomeinist seizure of power, Revolution Street will be the dividing line between two forces fighting for the country’s future. Under an informal deal negotiated between the authorities and the opposition, two rival marches will be held to mark the...