Depends on how complete the victory is. If Lee suffers more casualties and still escapes with the bulk of his army, it probably means that the rest of the war is fought more conservatively on the confederate side. If a substantial portion of Lee's army is captured, it's worth remembering that the AoNV at Antietam fought with almost half the men it later fights at Fredericksburg. It's a serious blow but it's possibly not an instant war ender.