Complex sentence: In the US, slavery was progressively abolished by northern states, but not until the Emancipation Proclamation of 1863 during the American Civil War, and the Thirteenth Amendment of 1865, across the whole country.
Split into simpler sentences: In the US, slavery was progressively abolished by northern state. But it was only the Emancipation Proclamation of 1863 during the American Civil War, and the Thirteenth Amendment of 1865, that slavery was abolished across the whole country.