Complex sentence: By 1915, he was working at the Division of Mexican Affairs as a clerk, where he was promoted as a clerk to'Class One'; in 1916, to'Class Two'; and in 1918, to Class Three'.
Split into simpler sentences: By 1915, was working at the Division of Mexican Affairs as a clerk, where he was promoted as a clerk to'Class One'. In 1916, to'Class Two'; and in 1918, to Class Three'.