The 12th of Tammuz, besides being the birthday of the previous Rebbe, Rabbi Yosef Yitzchak Schneerson, of blessed memory (1880-1950), is the day he was released from prison in the former Soviet Union in 1927. The reason for his arrest? Spreading Judaism.
Throughout the history of Russian Jewry, the spiritual and physical (literally) sacrifice made by the Chabad Hasidim to preserve Judaism is remarkable. From its beginnings (around 1780) to the present day (with the hundreds of institutions operating throughout Russia, Ukraine, and neighboring countries), Chabad Lubavitch has been utterly dedicated to the preservation of Judaism.
When the previous Rebbe came to the United States (in 1940), one of the first things he said was (in Yiddish): "America is nisht andersh" - America is no different.
The idea behind these words is that in America, that is, in the New World, in this new beginning (after the destruction of Europe at the end of the Second World War), we must not believe that sacrifice is no longer necessary to live and spread Judaism. In Europe it was clear: either one sacrificed oneself or one was lost as a Jew. In America, being a free country, things were easier. This is what the Rebbe teaches us: there should be no differences in a person's attitude toward Judaism according to their circumstances. The sacrifice must remain the same.
The only way to preserve (and grow) as a people is through sacrifice. Sacrifice for the proper education of our children (with Jewish principles and values), sacrifice for the observance of Shabbat, for the Kashrut of food, sacrifice for the study of Torah, and for spreading the Oneness of God throughout the universe.
The 12th of Tammuz teaches us that, regardless of the odds and appearances, those who walk the path of Torah and its mitzvot triumph. And the fundamental condition for that triumph is sacrifice.