Complex sentence: Reynolds started his career as a coach with St. Gallen in 1912, after a 2 year stay he left to become coach of the German national football team, but the outbreak of World War I prevented that and he moved to The Netherlands instead.
Split into simpler sentences: Reynolds started his career as a coach with St. Gallen in 1912, but after a two year stay he left to become coach of the German national football team. The outbreak of World War I prevented that, and he moved to the Netherlands instead.