A blanket party is when a group of soldiers kidnap one of their peers while he’s sleeping. They cover him with a blanket and beat him up, then put him back in bed.
My informant spent a few years in the Marine corps in the late 1970s and early 1980s. Blanket parties were done whenever the entire platoon was punished for something that was the fault of one soldier. For example if one guy was running extremely slowly and the DI (drill instrustor) made everyone an extra 5 miles because of it. My informant said that Blanket parties are brutal, but they proved to be very effective in making the platoon move and behave as a team. Even the most ornery soldiers became easier to work with.
This ritual is similar to a rite of passage on the Marine base that my informant was stationed, because almost everyone has caused problems for the rest of their group by messing up or making an obvious mistake. A balnket party is a punishment to remind the person being punished that they are not in charge and not to do anything that would purposely cause the rest of his platoon anguish. After more time has passed and the soldiers get to know each other, a mistake made by a soldier probably won’t result in them “getting a Blanket Party”. This practice is not only a rite of passage ritual, but it also unites the soldiers under the same identity.