Leading posek and president of Agudath HaRabbonim
Rabbi Moshe Feinstein on Eruvin in Manhattan, Brooklyn, and Large Cities
Rav Moshe examined New York eruvin over several decades, from the proposed Manhattan enclosure to later projects in Flatbush and Boro Park. His teshuvos discuss reshus harabim, shishim ribo, city walls, public roads, and the effect of visible carrying. In his practical rulings, he did not permit the Manhattan or Brooklyn projects brought before him.
Manhattan as one large city
Orach Chaim I:138–139 considers Manhattan's public roads together with the city's population and proposed walls. Rav Moshe does not treat a neighborhood route as though it served a separate town; the roads belong to the city used by the larger public.
The island's rivers and structures created possible grounds for enclosure, but bridges, tunnels, entrances, and public ways had to be repaired according to halacha. Rav Moshe concluded that the proposal before him could not be relied upon.
Yerushalayim, Manhattan, and visitors
In I:139 §5, Rav Moshe suggests that an eruv was not established for Yerushalayim because visitors came from many places for the regalim and throughout the year. Seeing public carrying there, they might later carry at home where the required walls did not exist.
He applies the concern to Manhattan and nearby places that could not be repaired in the same way. In V:28 §§13–15 he restates the point more narrowly: it is not an automatic reason against an eruv in every ordinary city, but it may apply to modern New York, which draws visitors from throughout the world, and perhaps to Brooklyn if the proposed enclosure is otherwise free of defects.
His practical position on Manhattan
Rav Moshe's name appears on the Agudath HaRabbonim prohibition of 18 Sivan 5722 with Rav Aharon Kotler, Rav Yaakov Kamenetsky, Rav Gedalia Schorr, and Rav Chaim Bick.
When his nuanced responsum was later cited in support of the project, he wrote to Rabbi Norman Lamm (4 Nissan 5724) that the Manhattan eruv was prohibited in his opinion and that his name must not be used as support. In Orach Chaim IV:89, he again says that he could neither permit nor assist it, while acknowledging that he lacked authority to silence qualified Rabbonim who ruled leniently.
Flatbush was not a separate small town
Orach Chaim IV:87 responds to a report that Rav Moshe had permitted an eruv in Flatbush. He writes because the claim was false. IV:88 then explains the Brooklyn setting, using contemporary estimates of roughly three million residents and another one million commuters and visitors.
Those figures belong to the period of the teshuva. Their significance is the method: Rav Moshe considered the population whose city roads served them, not only the residents inside the proposed neighborhood boundary.
The later Brooklyn ruling
Orach Chaim V:28 returns to Brooklyn and Boro Park, including the city's public-domain status and the effect of normalizing carrying. Rav Moshe again rejects the suggestion that his earlier position had changed.
No eruv should be established in Brooklyn
In a signed Union of Orthodox Rabbis letter (17 Teves 5739), Rav Moshe states that an eruv may not be made in Brooklyn because he regards it as a reshus harabim de'oraisa. He specifically rejects reliance on the Boro Park project and warns against making an eruv in any Brooklyn neighborhood.
The letter is a direct practical ruling, written after the longer analysis in Igros Moshe. It is the clearest short statement of his position on the active Brooklyn proposal.
Why Rav Moshe is cited in Crown Heights
Rabbi Avrohom Osdoba and Rabbi Shlomo Segal cite the leading poskim headed by Rav Moshe when ruling that no valid eruv exists anywhere in Crown Heights. His teshuvos supply the New York and Brooklyn analysis; Rabbi Zalman Shimon Dworkin's printed name on the 5739 proclamation connects that large-city ruling to Lubavitch rabbinic leadership.
The direct instruction for the neighborhood comes from the Crown Heights Beis Din: carrying remains prohibited on Shabbos and Yom Kippur.