The 101st/MC/VIW alliance was a group set up by the leaders of the repective clans. VIW already had an alliance with MC, and when the 101st Regiment joined up with VIW, they also joined with MC.
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Unlike the WJC, the 101/MC/VIW alliance is reletively binding. If a clan engages in war with another clan, all of the three clans are part of it.