Complex sentence: Seward openly suggested in September 1864 that if the Confederate States relented on the question of independence, the question of slavery would fall "to the arbitrament of courts of law, and to the councils of legislation".
Split into simpler sentences: Seward openly suggested in September 1864 that if the Confederate States relented on the question of independence. The question of slavery would fall "to the arbitrament of courts of law, and to the councils of legislation".