Due to his sympathy for "the underdog", Flex, like many other Germans of his time, sympathised with the Boer Republics in their battle against the British Empire.

In his following works for the stage, social problems form the core, as in Lothar, Die Bauernführer, Das heilige Blut, and Der Kanzler Klaus von Bismarck.

[4] Despite weak ligaments in his right hand, Flex volunteered for the Imperial German Army in his mother's birthplace of Rawitsch and was assigned to the 50th Infantry.

"[6] While going through officer training at Posen in early 1915, Flex met Ernst Wurche, a fellow member of the Wandervogel youth movement.

His epitaph was a quote from one of his works Preußischer Fahneneid ("The Prussian Military Oath" written in 1915): (Translation: His Wanderer zwischen beiden Welten was published in 1916, by Verlag C. H. Beck, and was well received.

[10] Tim Cross compares Flex's posthumous popularity and the idealization of his wartime death with the similar cultus surrounding English war poet Rupert Brooke.

[13] As a song, Flex's poem Wildgänse rauschen durch die Nacht gained popularity with the Wandervogel youth and was well known and sung in Germany until the 1970s.

Wer auf die preußische Fahne schwört[,] hat nichts mehr[,] was ihm selber gehört[.

][14] Translation: For the Kaiser and the Empire The Royal Prussian Lieutenant Walter Flex fell at Peudehof on 16 October 1917, born in Eisenach on 6 July 1887.