‘Ungentlemanly Warfare’ examines the impact of irregular warfare on strategic thinking in the New Zealand Wars (1845–1872). Britain entered the period as an eighteenth-century force reliant on European methods of war. In contrast, Māori were an experienced bushfighting force depleted on manpower but experiencing a period of military innovation. This interaction led to an important exchange of strategic ideology and methods. This thesis tracks how Māori, British, and colonial forces developed and exchanged irregular warfare practices across three regions and time periods. The Northern War exposed the shortcomings of conventional military conduct. Māori leadership altered their strategic approach and pursued a strategy of elastic offence. In contrast, British leadership persisted with their ineffective conventional means of warfare. In the Waikato War, British strategy was influenced by the success of colonial irregulars. British strategic and operational methods changed with the inclusion of multi-echelon warfare and the indirect approach. Kīngitanga leadership improved their combat capacity through guerrilla warfare, decentralised command, and a grand strategy of elastic offence. The Taranaki and West Coast campaigns highlighted the relationship between the use of irregular warfare and military success in New Zealand. Both sides experienced success and failure across numerous conflicts, the defining factor being the active use of irregular warfare. These case studies reveal the significance of irregular warfare strategies and tactics to military success in colonial New Zealand. They also reveal opportunities and challenges to military adaptation in Māori, British, and colonial martial cultures of the nineteenth century. ‘Ungentlemanly Warfare’ provides a unique approach to the examination of the New Zealand Wars conflict. It is an operational history that draws upon methods and aspects of the ‘war and society’ approach to military history, especially the use of Māori scholarship, evidence, and theory. The thesis’ chapters consider strategic and tactical decision-making across the numerous conflicts within the period. To do so, I provide close readings of primary evidence, including military communication and reports, early histories, and first-person accounts, to identify the roles of irregular warfare and strategic adaptions in the New Zealand Wars. ‘Ungentlemanly Warfare’ analyses the impact of irregular warfare on the strategic thinking of military leaders within the New Zealand Wars. I argue that irregular warfare strategies and tactics played a significantly larger role in the outcome of events than previously acknowledged. This study illustrates the importance of guerrilla warfare and the strategy of elastic offence to Māori strategy. In contrast, it highlights the evolution of British military thinking from conventional conservatism to a multi-echelon force led by irregular warfare units and tactics. ‘Ungentlemanly Warfare’ argues that irregular warfare was central to military conflicts in nineteenth-century New Zealand.