"Archer and hand infantry hitpoints increased."
In-game description

Infantry Breastplate is a technology in Age of Empires III that can be researched by European civilizations at the Arsenal; Iroquois and Sioux can also research it at the Longhouse and the Teepee respectively once the New Ways Home City Card has been sent. Once researched, it increases the hit points of melee infantry and archers by 10%.

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