Complex sentence: The first killing centre equipped with stationary gas chambers modelled on technology developed under "Aktion T4" was established at Bełżec in the General Government territory of occupied Poland; the decision preceded the Wannsee Conference of January 1942 by three months.
Split into simpler sentences: The first killing centre equipped with stationary gas chambers modelled on technology developed under "Aktion T4" was established at Bełżec in the General Government territory of occupied Poland. Notably, the decision preceded the Wannsee Conference of January 1942 by three months.