Simplify the following text for a Grade 7 native English-speaking person.
Original: The family had originated in Poland and had lived in Congress Poland ( the part of Poland then within the Russian Empire ) but had been displaced by World War I. Soon after Leonid 's birth , the family returned to Warsaw , Poland .
Simplified: His family is Polish and had lived in Congress Kingdom ( the part of Poland then in the Russian Empire ) but they moved to Russia to escape World War I. Soon after Professor Hurwicz was born , the family returned to Warsaw , in Poland .