Shneur Zalman Shazar (born Rubashov; "Shazar" is formed from his initials) was a scholar, writer and Labor Zionist leader, and the third President of the State of Israel, serving two terms from 1963 to 1973. He was born in 1889 in Mir, in the Minsk province of the Russian Empire, into a Chabad chassidic family, and received a cheder education under Chabad influence. As a young man he joined the Poalei Zion movement, and he later became a leading figure in the Histadrut and Mapai. He served in the First to Third Knessets and was Israel's first Minister of Education and Culture (1949–1951), responsible for the Compulsory Education Law of 1949.
Shazar remained attached to Chabad throughout his life. He helped the Previous Rebbe, Rabbi Yosef Yitzchak Schneersohn, in founding Kfar Chabad, and later developed a close and long-lasting relationship with the Rebbe, both in person and through extensive correspondence that began well before his presidency. In Israel he was known as "the Rebbe's chassid." As president he visited the Rebbe at 770 Eastern Parkway in 1966 — an unprecedented step for a sitting head of state, which he took despite warnings that it might cost him politically — and again in 1971, and he took part in the Rebbe's farbrengens, including after his presidency.
A partner in building Kfar Chabad. Many letters concern Kfar Chabad. The Rebbe thanked Shazar for his dedicated work on its behalf and asked him to keep using his influence for it (IK 5580, 8661, 8665): expanding it with planned growth and housing solutions (IK 5744, 5767, 6011), strengthening it after the tragedy that struck it, with real action rather than words (IK 4309), sending student-shluchim there (IK 4554), and seeing it as both a refuge for Jews in need and a center for spreading Chassidus (IK 8930). He also urged swift progress on establishing a second Chabad village (IK 7398, 7430, 7475). On naming the Kfar Chabad synagogue after Shazar, see IK 8850.
Chassidus and scholarship. The letters reflect Shazar's lifelong interest in Chassidus and Chabad history. The Rebbe encouraged his research on the works of the Alter Rebbe, stressing care with authentic manuscripts (IK 2651, 3060, 8644), sent him newly printed maamarim and reflections on chassidic history (IK 9004, 9207), and guided his study — continuing after Tanya with Torah Or, and translating learning into action (IK 7084, 6182). He also encouraged Shazar's support for spreading Chassidus and Tanya study, including in Rome (IK 6025), and urged him to attend the Yud-Tes Kislev farbrengen in Kfar Chabad (IK 7138).
Public responsibility. The Rebbe congratulated Shazar on his election as president, reminding him that a new position brings both greater responsibility and Divine help (IK 8629). He asked him to use his influence on behalf of Jewish youth and Torah values in Israel (IK 4728, 8383, 8446), to keep Jewish education free of political agendas (IK 7960), to advocate for new immigrants (IK 8323), and to act against altering sacred texts under academic pretexts and against missionary influence (IK 8785). He also stressed unity beyond political divisions, since every Jew inwardly wants to be connected to Torah (IK 7660).
Tone of the relationship. The letters are warm and personal. The Rebbe sent Shazar blessings for Pesach, Shavuos and the chassidic holidays, often with a teaching drawn from the season (IK 2591, 5375, 6297, 8714, 9674), and wished him well in his health so he could continue his work (IK 8997, 9009).
Sources: Jewish Virtual Library; Wikipedia; Chabad.org ("The Rebbe and President Shazar"); and the Rebbe's letters cited above.