Complex sentence: Menten travelled back to the Netherlands in 1939, when Russia invaded eastern Poland, and returned in 1941 after the Nazi counter-occupation--this time as a member of the SS. Menten was involved in the massacre of Polish professors in Lviv and robbery of their property.
Split into simpler sentences: Menten travelled back to the Netherlands in 1939, when Russia invaded eastern Poland. He returned to Poland in 1941 after the Nazi counter-occupation--this time as a member of the SS. Menten was involved in the massacre of Polish professors in Lviv and robbery of their property.