The argument: The halacha is that one cannot enclose a Reshus Harabim using a tzuras hapesach (a wire eruv), and it requires actual walls. I would like to explain why I hold that Crown Heights is a Reshus Harabim and therefore a tzuras hapesach eruv cannot permit it.
The rebuttal: It’s a shame that such a superficial statement was said from the get go. The Alter Rebbe maintains that a tzuras hapesach would reclassify a reshus harabbim as a reshus hayachid and only me’d’rabanan is there a requirement of delasos. Not to take this into account from the beginning is simply inexcusable. Moreover, Brooklyn is encompassed on three of its sides by actual walls.
The argument: Many opinions consider a street that is 16 amos wide (24 feet) to be a Reshus Horabim. The Alter Rebbe says (345:11,) that in such a case כל ירא שמים יחמיר לעצמו. For us Chasidim, the Alter Rebbe’s words are enough (the Tzemach Tzedek writes the same in Chidushim on Eiruvin). There are other opinions that there must also be 600,000 people for it to be considered a reshus harabim min hatorah. That exists in Brooklyn. This is the psak of R. Moshe Feinstein and R. Zalman Shimon Dvorkin. Even if you’ll find a contemporary Rabbi who claims otherwise – it is irrelevant. You can always search on the internet and find someone who will give you a heter for anything.
The rebuttal: If the Alter Rebbe’s words are enough for us Chasidim, how then could Rav Heller omit that Rav Avraham Chaim Naeh zt”l states (Kuntrus HaShulchan, p. 36 note 69) that it is probable that the Alter Rebbe never penned the words כל ירא שמים יחמיר לעצמו? Another, glaring omission is that the Rebbe maintained that the Alter Rebbe accepted the criterion of shishim ribo (see Igros Kodesh, Vol. IX, p. 41, 165). Furthermore, the Rebbe added there that the Bais Av (2:5:3) lists more than thirty Rishonim who accept shitas Rashi. The Bais Av is disagreeing with the Mishnah Berurah and asserts that a yorei shomayim does not need to be stringent. The Bais Av argues that we now know that the overwhelming majority of Rishonim uphold that in order to classify a street as a reshus harabbim it would need to be 16 amos wide and also include shishim ribo traversing therein.
Therefore, even though the Tzemach Tzedek states that a ירא שמים יחמיר [because many Rishonim do not accept shitas Rashi] since the Rebbe maintains that the Alter Rebbe accepted the criterion of shishim ribo for all and that we now know that the majority of Rishonim uphold the fundament of shishim ribo, it would be acceptable for a yorei shomayim to be lenient. Moreover, today with the publication of many more manuscripts of the Rishonim we can say that the Bais Av’s list has been superseded; we now know of over fifty Rishonim (and four Geonim) who accept the criterion of shishim ribo (mostly of Ashkenazic origin) and thirteen who do not (all of them of Sefardic origin). Thus, there is no doubt that a yorei shomayim can rely on shitas Rashi l’chatchilah.
Rav Heller declares, without delving into the matter, that the criterion of shishim ribo is satisfied in Brooklyn, and that this is the p’sak of Rav Moshe and Rav Dvorkin.
However, even if we were to accept that the kol korei which Rav Dvorkin signed on to is legitimate, the kol korei does not give any reason for opposing an eruv in Brooklyn. Who says that Rav Dvorkin signed because he supposed that the borough was classified as reshus harabbim of shishim ribo?
Regarding Rav Moshe’s opinion, even if Rav Moshe issued a p’sak, there is no doubt that his understanding of the criterion of shishim ribo differed greatly from the Alter Rebbe. While the Alter Rebbe maintained that shishim ribo is conditional of the street (see the Alter Rebbe’s Shulchan Aruch, 363:44) Rav Moshe upheld that it is conditional to an area of twelve mil by twelve mil (Igros Moshe, O.C. 1:139:5). Therefore, it is difficult to understand how Rav Moshe’s shita can obligate Chabad.
Moreover, notwithstanding the claim of the Flatbush kol korei, Rav Moshe never issued a p’sak din barur regarding Brooklyn, since his chiddush ¾ which he admitted was not the simple reading of the Shulchan Aruch (ibid., 1:139:5) ¾ was not mentioned in the Achronim, and moreover, it was obvious that the Aruch HaShulchan would not agree with him (ibid., 4:87).
Consequently, even Rav Moshe would admit that it not just, “someone from the internet who will give you a heter,” to establish an eruv in Brooklyn since as Rav Moshe admits [and the overwhelming majority of Achronim concur] the simple understanding of the Shulchan Aruch is that the criterion of shishim ribo is conditional of the street, and there is no street in Brooklyn where 600,000 people traverse any section of it on a daily basis.
I should add that since Rav Heller cites Rav Shlomo Miller, he should know that Rav Miller maintains that the parked cars themselves serve to minimize the width of the roads, and therefore, the streets are possibly not considered 16 amos wide (see The Laws of an Eruv, p. 60 note 55).
 Of course some are going to argue that since Rav Naeh states that the phrase כל ירא שמים יחמיר לעצמו was added by the brother [Maharil] of the Alter Rebbe, it would obligate those in Chabad to follow this dictum. However, this is missing the point. If the Alter Rebbe did not write this phrase himself, it indicates that he was not of the opinion that it was incumbent on all to be stringent regarding the criterion of shishim ribo. Rav Naeh suggests that if the Alter Rebbe’s brother was the one who added this phrase, it is probable that the Maharil saw this stringency in the Alter Rebbe’s household (and that is possibly why it may be notated in his ksav yad of the Alter Rebbe’s Shulchan Aruch), but this does not sanction that this stringency is for all yorei shomayim to follow (with this argument Rav Naeh answers why the Alter Rebbe in his Kuntres Achron, 252:2 only mentions that there are those who are stringent and, like the Taz, does not use the phrase, “all yorei shomayim should be stringent on themselves”). [It is well known that not all insertions made by the Maharil where accepted in Chabad; for example, see Siddur Rabbeinu HaZakan, p. 624. Furthermore, there are stringencies that were condoned in the Bais Harav but were not suggested for all; for example see Sefer HaMinhagim, p. 74 note 8.] I have also seen an argument that the fact that the Tzemach Tzedek cites his grandfather as upholding this stringency demonstrates that it must have been written by the Alter Rebbe. However, this is fiction. The Tzemach Tzedek does not write that his grandfather penned the phrase כל ירא שמים יחמיר לעצמו. The Tzemach Tzedek argues for this stringency from his personal opinion. On the contrary, the fact that the Tzemach Tzedek does not cite his grandfather on this issue is proof that Rav Naeh is correct to say that this phrase was penned by someone else.
 Of course, there are those who would argue that the Rebbe only mentioned that the Alter Rebbe upheld the criterion of shishim ribo, but the Rebbe never said that a yorei shomayim does not need to be stringent. These arguments are suggested by those who made up their minds prior to learning through the inyan. The Rebbe penned these ha’aros on Rav Tzvi Eisenstadt zt”l kuntres; the Rebbe added that the Alter Rebbe should be included with the other Achronim who paskend like shitas Rashi. The Bais Av that the Rebbe cited here was arguing with the Mishnah Berurah who admitted that the minhag was to follow shitas Rashi, but since most Rishonim do not accept the criterion, a yorei shomayim should be stringent. However, the Bais Av argues that the Mishnah Berurah is incorrect since the majority of Rishonim [and Geonim] accept shitas Rashi, hence even a yorei shomayim does not need to be stringent [and like the Taz states (345:6) of course one can always be machmir, but it is not a matter of yiras shomayim]. Therefore, if the Alter Rebbe actually penned this stringency, the Mishnah Berurah’s shita would be similar to the Alter Rebbe’s shita. Consequently, the fact that the Rebbe did not indicated here that unlike the Bais Av the Alter Rebbe would agree with the Mishnah Berurah that a yorei shomayim should be stringent, demonstrates that the Rebbe maintained that the Alter Rebbe would not require that a yorei shomayim needs to be stringent (either because the Alter Rebbe never penned these words or because we now know that the overwhelming majority of Rishonim uphold shitas Rashi).
 Furthermore, if we explore Rav Moshe's personal reservations regarding establishing an eruv in Brooklyn, we see that it is apparently based on misinformation which was provided to him at the time such as Brooklyn has more than three million residents over a twelve mil by twelve mil area (Igros Moshe, O.C. 5:28:5, 5:29), [the population required to classify the area as a reshus harabim of shishim ribo, according to Rav Moshe] while in fact, according to census figures, the entire Brooklyn [which is larger than twelve mil by twelve mil (ibid., 4:87-4:88)] only has 2,504,700 residents.
Additionally, Rav Moshe wrote (ibid., 5:28:5) that he was not certain if Brooklyn was bounded by mechitzos [if Brooklyn was encompassed by mechitzos, it would not be classified as a reshus harabim d'Oraysa (Ibid., 1:139:3)]. In fact, it has been established that Brooklyn is encompassed by man-made mechitzos on three of its sides; hence, an eruv can be erected in any part of Brooklyn even according to Rav Moshe’s shita (these mechitzos were confirmed by the following rabbanim: Members of Hagaon Harav Yechezkel Roth shlita’s Bais Din (see Emek HaTeshuvah, 5:19); Hagaon Harav Shlomo Gross shlita, Belzer Dayan of Boro Park, and Hagaon Harav Tuvia Goldstein zt”l sent a select group from his kollel Emek Halacha).