Rav Moshe Feinstein's Brooklyn Rulings
Rav Moshe returned to Brooklyn eruv questions repeatedly and rejected reports that his position had changed.
The New York foundation
Igros Moshe Orach Chaim I:138–139 develops Rav Moshe's early New York analysis. He examines the scale of Manhattan, its public streets, possible walls, and the conditions required before a city eruv could be permitted.
That framework becomes the basis for his later treatment of Brooklyn. The borough is not assumed to be a collection of small independent towns.
Flatbush was not permitted
Orach Chaim IV:87 corrects a public report that Rav Moshe had permitted a Flatbush eruv. He states that the report was false and that his position had not changed.
The following teshuva continues the Brooklyn analysis, including the relation of a neighborhood to the population and roads of the larger city.
The later Boro Park discussion
Orach Chaim V:28 returns to large-city and Brooklyn questions. It includes the concern that visible carrying can mislead visitors who later return to a place where no comparable eruv exists.
Rav Moshe's later writing therefore continues both sides of his approach: the technical status of the city and the public consequences of the practice.
The 17 Teves 5739 letter
When a Brooklyn eruv was again being promoted, Rav Moshe issued a signed letter with the Union of Orthodox Rabbis. He stated that the proposed eruv should not be established and continued the public position expressed in his responsa.
The letter is short because it gives a practical ruling against an active proposal, not because the underlying analysis was slight.