When World War I began Tersztyánszky, commanding IV Corps, served as part of 2nd Army in the Balkans theatre. Later that year the unit was transferred to the Eastern Front to fight in Galicia and Poland. In late spring 1915 he was given command of the short-lived Army Group "Tersztyánszky", which soon became the 3rd Army, in Serbia. However, later that year he lost command after clashing with Hungarian Prime Minister István Tisza.