The Old and New Testament are the same story. The entire Bible tells about how God made a perfect world, man messed it up, and how a God of both justice and mercy planned long ago to pay our penalty with his own blood. The story is not over. For now the world will continue to allow for the redemption of many, but someday the Lord will pour out his wrath on all his enemies. We are hardly done with holy warfare. Not at all.
It is said that the Old Testament is the New Testament concealed, and the New Testament is the Old Testament Revealed.
The Bible teaches that the Messiah prophesied in the Old Testament came first as a suffering servant, to sacrifice himself for our sins and to conquer death. But when He comes again, He comes in power, and he will lead and army of his people. He will come ready for battle, and EVERY knee shall bow.
The God of the New Testament and the God of the Old Testament are one and the same.