From 1894 to 1907 Zentgraaff served in the KNIL or Royal Netherlands East Indies Army, where he attained the rank of 'sergeant writer'. He fought in 1895 and 1896 in the Aceh War and participated in the Boni expeditions of 1897 and 1905. The Aceh War, also known as the Dutch War or the Infidel War (1873-1914), was an armed military conflict between the Sultanate of Aceh and the Netherlands which was triggered by discussions between representatives of Aceh and the United Kingdom in Singapore during early 1873. The war was part of a series of conflicts in the late 19th century that consolidated Dutch rule over modern-day Indonesia.