AIntroduced in UC 0075, the MS-05B Zaku I was the Principality of Zeon's first true mass-production model mobile suit. Although designed as a front line combat unit, the Zaku I was soon replaced by the improved MS-06F Zaku II and would be replaced as the mainstay of Zeon's mobile suit forces. Early during the One Year War of UC 0079, most Zaku Is were relegated to menial construction and supply duties, although a few did continue to serve in combat roles. During the war's opening days, Zaku Is were also used in other combat-related work roles, such gassing of colony populations, attaching booster engines to space colonies to hurtle them towards Earth, and so on. It wasn't until the last months of the war that Zeon, desparate after facing losses in Operation Odessa and Operation Star One, would move the old Zaku Is back to the front lines to reinforce their faltering main forces.