Due to requests, I am reformatting the Questions and Answers Regarding the Flatbush Eruv posts.
Q: Why do we need an eruv? Can't we observe Shabbos properly without one?
A: The Chasam Sofer  states that it is not possible for an individual to ensure that all the members of his household do not inadvertently carry on Shabbos, and therefore, the construction of an eruv is the obligation of every rav. In light of this Chasam Sofer it follows that one cannot assert that there is no need for an eruv. Furthermore, an eruv helps minimize chilul Shabbos by our Jewish brethren who are unfortunately not religious and carry on Shabbos without an eruv. Additionally, an eruv helps to increase our oneg Shabbos, e.g., families with young children, the elderly, and the infirm are no longer confined to their homes. An eruv is invaluable on Yom Tov, as well, as it allows us to carry items that are not needed e.g. extra keys on a key chain. Furthermore, it is a mitzvah to erect an eruv. Rav Moshe Feinstein zt”l, when he recognized that an eruv could be erected, saw a great need for it.
Q: Is there a precedent for constructing an eruv in a large city?
A: Yes. The custom of erecting eruvin has existed for generations. Prior to WWII, most European cities with Jewish populations such as Brisk, Lodz, Odessa, Paris, Radin, Vilna, and Warsaw all had eruvin. The minhag of our ancestors was to erect eruvin. Therefore, not to establish an eruv is a departure from our minhag.
Almost all large cities, prior to WWII erected eruvin when the civil authorities allowed them to. The following large cities with populations of more than 600,000 established eruvin: Warsaw, Lodz  [Warsaw, from at least the year 1900, had a population of more than shishim ribo on one side of the Wistula River which divided the city in two ― the larger side known as Warsaw and the significantly smaller side known as Praga; Lodz had a population of more than shishim ribo since the year 1931 ], Odessa, Manchester, St. Louis, and New York in 1905. Even Rav Moshe Feinstein zt”l acknowledges that in the past eruvin had been erected in cities with populations exceeding shishim ribo.
Bnei Brak [including it’s interconnected neighborhoods] and Yerushalayim, both of which contain more than shishim ribo, are enclosed with eruvin under the hashgacha of Rav Landau shlita and the Eidah Hachareidis.
It is important to note that in 1938 the Achiezer, Rav Chaim Ozer Grodzinski zt”l, and the Chazon Ish zt”l allowed the establishment of an eruv in Paris  [1936, population 2,829,746 ] with exactly the same conditions as in Brooklyn: three mechitzos, omed merubeh al ha’parutz.
Q: We’ve never had eruvin in New York before, why start now?
A: Most people are unaware that there was an eruv established in New York City [Manhattan] as far back as 1905 with the approval of some of the greatest poskim of that generation. That there was opposition ― spearheaded by laypeople and not rabbinical authorities ― to the establishment of a Flatbush eruv in 1979 does not preclude the erecting of an eruv now. Moreover, even Rav Moshe Feinstein zt”l in one of his last teshuvos regarding eruvin realized that today ― where halachically permissible ― there is a great need for eruvin. The fact that most poskim and, in our opinion, even Rav Moshe would rule that an eruv is halachically permissibly today makes the establishment of an eruv an obligation. Additionally, in Brooklyn there are numerous private eruvin, many of which are not halachically correct, and a community eruv would prevent inadvertent chilul Shabbos.
 Orach Chaim 99.
 Nefesh Chayah, siman 25 and Bais Av, 2:1:25.
 Perishah, O.C. 395:1.
 Shulchan Aruch, O.C. 518:1.
 Tur, and Shulchan Aruch, O.C. 366:13, 395:1; for proof that it’s a requirement for a city as well, see BeHag, Perek Hador and Chasam Sofer, O.C. 99.
 Igros Moshe, O.C. 1:139:5, 4:86, 5:29.
 Mishmeres Sholom, 24:10.
 Rocznik Statystyczny Warszawy 1921 i 1922, 1924 page 14.
 Encyclopedia Judaica, 1996 vol. 11 page 426.
 Divrei Malkiel, 3:14-18, 4:3; Tikkun Shabbos, and Tuv Yehoshua.
 Introduction Bais Av vol. 2 and Encyclopedia Britannica, 1911 vol. 17 page 547.
 Tikvas Zechariah and Encyclopedia Britannica, 1911 vol. 24 page 24.
 Oznei Yehoshua, 1:18; Tirosh VaYitzhar, siman 73; Eruv V’Hotzaah, and US Census 1900.
 Igros Moshe, O.C. 4:87; see The Community Eruv page 30.
 Hagaon Harav Yisroel Yaakov Fisher zt”l in Even Yisroel, 8:36 and Kinyan Torah, 4:40.
 Achiezer, 4:8.
 Encyclopedia Britannica, 1968 vol. 17 page 355.
 Igros Moshe, O.C. 5:29.