In August 1915, Otto Frank went into the German army. He was trained as a gunner. From the autumn of 1915 onwards, he was stationed at the Western Front. He was part of a so-called Lichtmesstrupp: a unit that detected and calculated the location of enemy guns and passed information about the trajectory on to the artillery.
In 1916, he took part in the Battle of the Somme, in which almost half a million German soldiers were killed. A year later, Otto was promoted to sergeant and the year after that to officer candidate.
Otto's brothers Herbert and Robert served in the German army as well. Their mother Alice Frank-Stern and sister Leni Frank were nurses in a military hospital.