Rav Moshe Feinstein's Responsa Concerning Manhattan
Rav Moshe addressed Manhattan across decades and repeatedly stated that he could not permit or assist the proposed eruv.
The foundational teshuvos
Igros Moshe Orach Chaim I:138–139 develops Rav Moshe's analysis of Manhattan's streets, population, walls, and possible enclosure. He treats the city as a major public-domain question and refuses to permit the proposed eruv.
Later teshuvos revisit changed proposals and arguments without abandoning that conclusion. The discussion extends into Brooklyn because the same large-city facts and public roads continue beyond Manhattan.
Respect for dissenting Rabbonim
Rav Moshe wrote respectfully about qualified Rabbonim who disagreed with him. He did not treat their support for the eruv as a reason for personal attack.
That restraint was sometimes presented as though he had accepted the project. In Orach Chaim IV:89 and in his direct correspondence, he distinguishes respect for the people involved from his own continued refusal to permit or assist the eruv.
Public rulings and the use of his name
Rav Moshe appears among the leading Rabbonim on the Agudath HaRabbonim ruling of 18 Sivan 5722. In 1964 he sent Rabbi Norman Lamm a telegram and letter objecting to the use of his name as support for the Manhattan eruv.
These are practical public acts consistent with the teshuvos: he did not permit the project and did not want the public told otherwise.