Complex sentence: From the extinction of the House of Than in 1603 Dahn was head office of a department of the diocese of Speyer until the french revolutionary wars in 1797/98, that is why the white cross was adopted from that diocese.
Split into simpler sentences: From the extinction of the House of Than in 1603 Dahn was head office of a department of the diocese of Speyer until the french revolutionary wars in 1797/98. That is why the white cross was adopted from that diocese.