In the sixth aliyah we return to some narrative of action in the story of the Jews in the desert, rather than Moshe's review of the Torah that we've had since the beginning of the book of Devarim. Here, G-d says to Moshe that the day of his passing away is approaching, and that Moshe should come with Yehoshua to the Mishkan. They go there and G-d appears to them and tells Moshe that after he passes away, the Jewish people will abandon G-d and stray after the alien gods of the land and evil troubles will befall them. Therefore G-d tells them to write the song contained in the next parsha as witness of why the troubles befell them. The Talmud (Sanhedrin 21b), derives the halacha for every Jew to write a Torah from this passage. The Rebbe instituted a very easy way to participate in this mitzvah, and at the same time unite Jews, and that is by every man, woman, and child buying a letter in a sefer Torah.