Why No Peace on Earth?

When the angels announced the birth of Jesus to the shepherds, did they also promise peace on earth for all men? If so, why haven’t we experienced peace since then? How do we explain the paradox that nearly all really great religious and political leaders have acknowledged the utter futility of war, while war is ongoing? Did early Christians fight in war? What did Christ teach us? Is the concept of righteous war in accordance with the Bible? Will we ever experience peace on earth?