Complex sentence: He gained virtual immunity from law enforcement through mastery of New York City politics at the Democratic political machine in Tammany Hall that ruled Manhattan virtually unopposed, controlling the city's police and bureaucracy that handed out the construction contracts and licenses.
Split into simpler sentences: He gained virtual immunity from law enforcement through mastery of New York City politics at the Democratic political machine in Tammany Hall that ruled Manhattan virtually unopposed. The political patronage of Tammany Hall controlled the city's police and bureaucracy that handed out the construction contracts and licenses.