Everyone has heard about how the Jewish world was torn asunder at the end of the 1700's as a result of the quarrels between Chassidim and Misnagdim. The Vilna Gaon felt that Chassidus was a danger to Torah True Judaism. Until this day there are some who would like to continue the battle.
A May'seh....
Rav Soloveitchik once attended and greatly enjoyed a fahrbrengen of the Lubavitcher Rebbe. Afterwards the Rebbe was heard saying that he is sure that in shomayim the Vilna Gaon and the Ba'al Ha'Tanya have made peace. The proof is that a student of the students of the Vilna Gaon attended a gathering of a student of the students of the Ba'al HaTanya.
Rav Soloveitchik commented in return that he thinks that peace was made between the two warring parties when Hitler threw the followers of both into the gas chambers. He, yimach shmo, knew that there is really no difference. A Jew is a Jew.
[From the sefer Pninei HaRav]