Complex sentence: Members of the this religious group refused to serve in the German military or give allegiance to the Nazi government, as a result hundreds were executed, while an estimated 10,000 were sent to concentration camps where approximately 2,500 of them were killed.
Split into simpler sentences: Members of the religious group refused to serve in the German military or give allegiance to the Nazi government, for which hundreds were executed. An estimated 10,000 were sent to concentration camps where approximately 2,500 of them were killed.