The custom of utu, or reciprocation, led to a growing series of reprisals as other iwi realised the benefits of muskets for warfare, prompting an arms race among warring groups.[1] In 1821 Hongi Hika travelled to England with missionary Thomas Kendall and in Sydney on his return voyage traded the gifts he had obtained in England for between 300 and 500 muskets, which he then used to launch even more devastating raids, with even bigger armies, against iwi from the Auckland region to Rotorua.[5][2]