Complex sentence: On 19 January 1920, Polish military and political authorities marched into the city and a Polish administration was established, but unemployment and living conditions deteriorated, leading to a wave of strikes, starting in 1921.
Split into simpler sentences: On 19 January 1920, Polish military and political authorities marched into the city and a Polish administration was established. Unemployment and living conditions deteriorated, leading to a wave of strikes, starting in 1921.