During World War I, whilst still an MP, he served in the military as a Captain of the 1st Regiment of Life Guards, was injured at the First Battle of Ypres and commanded the mounted troops of the Guards Division from 1915–16.
British soldiers of the 19th Infantry Brigade made the earliest burials at the site in October 1914 during the First Battle of Ypres.
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During the Battle of the Yser, part of the First Battle of Ypres in World War I, Hendrik Geeraert opened the sluice gates on the mouth of the river Yser twice to flood the lower lying land, thus halting the German advance.