The Rebbe taught this discourse on Shabbat Mattot-Mas'ey in 1969.  The Sedra tells us that the sons of Gad and Reuben wanted to stay on the East bank of the Jordan.  They sought the spiritual, rather than to plunge into the activity of  entering the Land of Israel.  Moses only accepted this when they agreed to risk their lives to help the rest of the Jewish people conquer the Land.  Today each of us needs both aspects - the quest for the spiritual and also the readiness for self-sacrifice in making the practical world a dwelling for the Divine.