New Zealand's peak year for wartime mortality burden: the important role of the Battles of Messines and Third Ypres (Passchendaele) in 1917.

At a total of 5,547 deaths among New Zealand's military personnel, the year 1917 was the worst year from a mortality perspective in the country's military history. This year had a third of the deaths in the whole of the First World War for this military population. Major drivers of this mortality burden were the Battles of Messines and Third Ypres (Passchendaele) in June and October 1917 respectively. The contribution of disease deaths to the mortality burden was relatively small at 4.5%. Disease deaths were significantly more common in the Northern Hemisphere's winter months (p=0.007), and some may have been related to crowding.