He served as chair of the Council of Sages and one of three co-deans of the Ponevezh Yeshiva in Bnei Brak, along with Shmuel Rozovsky and Dovid Povarsky.

Later, in 1988, Shach criticized Ovadia Yosef, saying that, "Sepharadim are not suitable for leadership positions",[1] and subsequently founded the Degel HaTorah political party representing the Litvaks in the Israeli Knesset.

During this period he suffered considerable deprivation, living with inadequate sanitation and being compelled to wear tattered clothing and worn out shoes.

In early 1940, Shach's maternal uncle, Aron Levitan, helped him get emigration visas to the United States, but after consulting with Yitzchok Zev Soloveitchik and Grodzinski,[7] Schach decided to immigrate to Mandatory Palestine.

[8] Several years after the re-establishment of the Ponevezh yeshiva in Bnei Brak, Shach was invited by Yosef Shlomo Kahaneman to become one of its deans, and, after discussing the proposal with Soloveitchik, he accepted the offer.

Shach received semikhah (rabbinical ordination) from Isser Zalman Meltzer,[10] and served as chairman of Chinuch Atzmai and Va'ad HaYeshivos.

[citation needed] Other complaints included Hamodia publishing a series of articles based on the teachings of Rabbi Menachem Mendel Schneersohn (the Lubavitcher Rebbe).

[19] Following a visit by Shach in Jerusalem to the leading rabbis and halachic decisors of the day, Yosef Shalom Eliashiv and Shlomo Zalman Auerbach, in order to seek their support for the new party.

In a speech delivered prior to the 1992 elections, Shach said that Sephardim were not fit for leadership and aroused great anger among Sephardi voters.

Labor Party politician Yossi Beilin said Shach's speech set back relations between religious and secular Israelis by decades.

[22] Other secular Israelis, including residents of the kibbutz Ein Harod, were said to have found the speech inspirational, so much so as to bring them closer to religious practice.

[23] In 1985, four years after the Labor Party supported a liberalized abortion law, Shach refused to meet with Shimon Peres and said he would not speak with a "murderer of fetuses".

Shach often said that for true peace, it was "permitted and necessary to compromise on even half of the Land of Israel", and wrote that, "It is forbidden for the Israeli government to be stubborn about these things, as this will add fuel to the fire of anti-Semitism".

[41] In 1988, Shach denounced Schneerson as a meshiach sheker (false messiah),[42] and compared Chabad Hasidim to the followers of the 17th century Sabbatai Zevi,[43] branding as idolatrous Schneerson's statement referring to his father-in-law, the previous rebbe of Chabad, which he viewed as God's chosen leader of the generation, "the essence and being of God clothed in a body of the "Moses" of the Generation, as it was by Moses himself".

[47] In a lengthy attack on Joseph B. Soloveitchik (d. 1993) of Yeshiva University, Shach accused him of writing "things that are forbidden to hear",[48] as well as of "... endangering the survival of Torah-true Judaism by indoctrinating the masses with actual words of heresy".

[49] Shach resigned from the Moetzes Gedolei HaTorah ("Council of Torah Greats") following tensions between him and the Gerer Rebbe, Simcha Bunim Alter.

However, it would not be accurate to base the entire conflict on a renewal of the historic dispute between Hasidim and Mitnagdim which began in the latter half of the eighteenth century.

"[60] Chief Rabbi Yisrael Meir Lau said Shach's most important contribution were his efforts in restoring Jewish scholarship after the Holocaust.

[25] David Landau wrote that his "uniqueness lay in the authority he wielded", and that "perhaps not since the Gaon Elijah of Vilna, who lived in the latter part of the 18th century, has there been a rabbinical figure of such unchallenged power over the Orthodox world".

[62] Avi Shafran of Agudath Israel of America said: "His pronouncements and his talks when he was active would regularly capture the rapt attention of the entire Orthodox world.

Shach was survived by his daughter Devorah, who had nine children with Meir Tzvi Bergman, and his son Ephraim, who rejected the Haredi lifestyle[64] and joined the Religious Zionist movement.