Hitler originally wore the Kaiser moustache, as evidenced by photographs of him as a soldier during World War I. There is no agreement as to what year Hitler first adopted the toothbrush. Alexander Moritz Frey, who served with Hitler during World War I, said Hitler wore the toothbrush in the trenches after he was ordered to trim his moustache to facilitate the wearing of a gas mask. Cultural historian Ron Rosenbaum instead says Hitler did not wear it until late 1919; he was also photographed c.1921–1924 with a more traditional moustache in between wearing the Kaiser and toothbrush style.