Complex sentence: The Nazis intended to destroy the Polish nation completely and in 1941 Nazi leadership decided that in 10 to 20 years Polish state was to be fully cleared of any ethnic Poles and settled by German colonists.
Split into simpler sentences: The Nazis intended to destroy the Polish nation completely. In 1941, the Nazi leadership decided that the Polish state was to be fully cleared of ethnic Poles within 10 to 20 years and settled by German colonists.