At the beginning of the First World War, Philipp enlisted in the Hessian Dragoon-Regiment Nr. 24 along with his older brother Maximilian. They served first in Belgium where Maximilian was killed in October. In 1915 and 1916, Philipp served on the Eastern Front in what is now Ukraine. He held the rank of lieutenant (an extremely low rank considering his princely background) and was mostly responsible for the procurement of munitions. In 1917, he served on the Hindenburg Line, before returning to Ukraine where he experienced active combat and was wounded.