Shlomo Zalman Shragai was a leader of the religious Zionist movement (Mizrachi and Hapoel Hamizrachi). He served as the first mayor of Jerusalem after the founding of the State (1950–1952), and later headed the immigration department of the Jewish Agency. He came from a chassidic family and wrote extensively on religious and Zionist thought.
Shabbos on Israeli ships. Several letters concern the controversy over Israeli ships and organized sea voyages involving Shabbos desecration. The Rebbe urged him, as a communal leader, to protest firmly (IK 4531) and to ensure Shabbos observance on Israeli ships, noting that even advanced technology does not remove the problem (IK 4733, 6089).
Chassidus and Jewish life. The Rebbe thanked him for his articles and for his support of Kfar Chabad, and encouraged him to use his influence for Jewish causes (IK 6348). He wrote that Chassidus deepens, rather than diminishes, mitzvah observance (IK 7328), and that steadfast commitment overcomes every obstacle in spreading Judaism (IK 7700).
Recognizing G-d's hand. The Rebbe wrote that we must recognize G-d's hand in miraculous events rather than attributing success to human strength (IK 9384), and wrote about the joy of Tu B'Av and its connection to the redemption (IK 8495).
Sources: Wikipedia ("Shlomo Zalman Shragai"); and the Rebbe's letters cited above.