"Ambiguous statements questioning Germany's heavy responsibility for the breakout of World War II are unacceptable," said Guido Westerwelle.
Westerwelle's reaction comes after Erika Steinbach, a member of Chancellor Angela Merkel's Christian Democrats, suggested in a party meeting on Wednesday that Poland mobilized its troops months before Nazi German invaded the country on Sept. 1, 1939.
The Nazi invasion of Poland triggered the outbreak of World War II.
Westerwelle said such statements not only contradict historical facts, but are also damaging for the country's image.
Opposition lawmakers demanded that Merkel distances herself publicly from those remarks.
Merkel immediately rejected these remarks, said Volker Kauder, a senior party official.
Steinbach heads a group representing ethnic Germans who were expelled from Poland after the end of that war, and she is deeply distrusted in neighboring Poland.
Steinbach, meanwhile, announced that she would not seek another term in the party's 25-member board, German news agency DAPD reported.
"I represent the conservative ideas there, but I'm increasingly alone," German newspaper Welt quoted her as saying.