Simplify the following text for a Grade 4 native English-speaking person.
Original: Horst KÃ hler 's parents , ethnic Germans and Romanian citizens , had to leave their home in Bessarabia in 1940 during the Nazi-Soviet population transfers that followed the invasion of Poland and the Molotov-Ribbentrop Pact , which awarded Bessarabia to the Soviet Union .
Simplified: Horst KÃ hler 's parents had to leave their home in Bessarabia in 1940 .