AN UNLIKELY UNION: The Love-Hate Story of New York's Irish and Italians, by Paul Moses. NYU Press, 381 pp., $35.

Few American cities, with the possible exception of Chicago, do urban ethnic drama like New York. In the late 19th century and into the 20th, Gotham's Italians and Irish mixed it up in a prizefight for power, influence and access. Pint-size pol Fiorello La Guardia battled debonair James W. Walker in mayoral politics, while Frank Sinatra jousted with Bing Crosby for the hearts (and ears) of swooning fans everywhere. Such contests were emblematic of larger struggles between the two ethnic groups in the church, workplace, union office and police station.

In "An Unlikely Union: The Love-Hate Story of New York's Irish and Italians," former Newsday city editor Paul Moses charts their fractious fights, assimilation and eventual intermarriage in the postwar years. Brooklyn-born Moses is a product of these forces: the grandson of a Calabrian immigrant, he married a woman with Irish roots.

This is a story with a nominally happy ending. Without getting too Kumbaya about it, Moses shows how the Irish and the Italians were better off working together, not against one another. But such synergy was a long time coming. When Italians starting arriving en masse in the 1880s, the Irish were almost everywhere dominant. The Catholic Church became a site of contention: The more ecstatic worshipping style of the Italians was off-putting to the stern Irish bishops who set Church policy. In the workplace, Italians competed with the Irish in the construction and laboring trades. Irish union leaders fumed about cheaper wages paid to Italians. During the 1890s, Italian and Irish longshoremen duked it out on the docks in so-called "race riots."

But Italians started to unionize. Moses describes how, during the building of the New York subway in the early 1900s, union leader Tito Pacelli tried to get better pay for his workers, despite resistance from reluctant Irish-dominated locals. Throughout "An Unlikely Union," Moses describes a process that begins with suspicion that turns to grudging acceptance. Irish union leaders realized that the union movement was better off with Italians inside of it.

This was also true in the Irish-dominated police force. If the force looked suspiciously on Italian newcomers, it still needed Italian detectives to help bust the Black Hand, an extortion racket that targeted Italians. Moses details the fascinating exploits of Joseph Petrosino, one of the NYPD's first Italian detectives. Newspapers criticized William McAdoo, the Irish-born police commissioner, for not doing enough. In 1904, he created an Italian squad, led by Petrosino, to gather intelligence on shadowy crime networks. Still, Petrosino had to contend with testy colleagues who resented his rise. But racking up conviction after conviction, he became a headline-making star.

One prize remained for New York's Italian-Americans: the mayor's seat. In one of the book's most entertaining chapters, Moses regales us with the rise of La Guardia as he took on the Irish-dominated Tammany Hall Democrats and the colorful James W. Walker. La Guardia, a passionate reformer, would serve three terms as mayor from 1934 to 1945, building a powerful coalition of Italian and Jewish voters.

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For all their avowed differences, the Irish and Italians gradually came together. Moses argues convincingly that once the two groups mingled in churches, schools and other public realms, and started to share the same set of cultural norms, tensions eased. It is partly a story of assimilation, and partly a story of rising up the economic ladder. "In this world of frayed and raw racial and ethnic connections, the Irish and the Italians of New York offer a story of how peace was made," Moses writes.

Amen to that.