Original: Adolf Hitler, then a corporal in the German army, was ordered to spy on the DAP in September 12, 1919 during one of its meetings at the Sterneckerbräu, a beer hall in the center of the city.
Simplified: When Adolf Hitler was still a corporal in the German army, he was ordered to spy on the DAP during one of its meetings at the Sterneckerbräu on 12 September 1919.