The correct answer is when he made the Emancipation Proclamation
At first, the war was fought to preserve the union. The northerners wanted to keep the seceding states in the union and they went to war to stop their independence. It was during the war that he made the emancipation proclamation which exclaimed that all slaves in the seceding countries were free and were welcome to join the northern army and this led to the three amendments that were introduced after the war.