World War I broke out in Europe in 1914 and, though he was in his fifties, Stäckel saw it as his duty to support his country and joined the army as an officer, whilst still continuing with his university commitments.
World War I Nasira out sa Europa at sa 1914, kahit na siya ay sa kanyang ikalimampu, Stäckel nakita ito bilang kanyang tungkulin sa suporta ng kanyang bansa at sumali sa hukbo bilang isang opisyal, habang patuloy pa rin sa kanyang mga unibersidad commitments.