After the Yom Kippur War in 5734, spanning the months of Nissan to Tammuz, terrorists infiltrated Eretz Yisroel and killed ר”ל many Yidden. (See footnote for more information on the four main attacks.)
At that time, an urgent call from the Rebbe directed at all Jews was published that included an excerpt from a sichah (linked below). In this sichah, the Rebbe pointed out the importance of each and every Jew becoming actively involved in the five mivtzoim (that had been established at that point).
The Rebbe explained how these mivtzoim each have the unique power to protect Jews from harm. The Rebbe concluded the sichah by saying that a mitzvah done in one location protects not only the one who does it, but also all Jews wherever they are, even those who have not yet fulfilled the mitzvos!
See the sicha here
The four main attacks in 5734: