The New Zealand Expeditionary Force (NZEF) was the title of the military expedition sent from New Zealand to fight for Britain in World War I. Upon the outbreak of war, New Zealand immediately offered to provide two brigades — one of infantry and one of mounted troops — a total of 8,500 men. This contingent sailed for Australia within two months of the start of the war and then joined with the Australian Imperial Force in a convoy sailing for Egypt.

The NZEF was commanded throughout the war by General Alexander Godley , a British officer who in 1910, on the recommendation of Lord Kitchener, had been appointed as the commander of the New Zealand Defence Forces. New Zealand, like Australia, had a pre-war policy of compulsory military training but the NZEF was initially reinforced by volunteers only. Conscription was introduced on August 1, 1916 and by the end of the war 124,000 men — nearly half the eligible male population of 250,000 — had served with the NZEF. Of these, about 100,000 had been sent overseas.

The NZEF was closely tied to the AIF for much of the war. When the Gallipoli campaign began, the New Zealand contingent was insufficient to complete a division of their own so was combined with the Australian 4th Infantry Brigade to form the New Zealand and Australian Division, General Godley commanding. This division, along with the Australian 1st Division, formed the famous Australian and New Zealand Army Corps (ANZAC) under the command of General William Birdwood.

After the end of the Gallipoli campaign, the NZEF formed its own infantry division; the New Zealand Division which served on the Western Front for the rest of the war. General Godley was promoted to a corps command and given II Anzac Corps, which contained the New Zealand Division. From 1916 until the formation of the Australian Corps in 1918 (made up of the five Australian divisions) there were always two "Anzac" corps — I Anzac Corps and II Anzac Corps — despite the fact that there was only one New Zealand Division to go around.

The mounted arm of the NZEF remained in Egypt and, combined with the 1st and 2nd Australian Light Horse Brigades, made up the Anzac Mounted Division which served through the Sinai and Palestine campaign.

The New Zealand Expeditionary Force was finally disbanded on December 31, 1921.