In 1914, he was conscripted and joined the Kaiserliche Marine serving on minesweepers and rising eventually to the rank of "Mate". In 1917 he was awarded the Iron Cross. In 1918 took part in the November Revolution at Cuxhaven. Returning to Munich, Beimler joined the Spartacus League and in the chaotic period following the armistice, during which there were several revolutionary governments in Germany, he supported the Munich Soviet ("Räterepublik").