Where in the OT does it promise the forgiveness of sins for fighting and dying in battle?
ealgeone:
In Maccabees 2: 12:46, Jewish men who had fallen away (embraced pagan gods, etc.) fought to defend the Temple against a non-Jewish army (Greeks), and in doing so they died as martyrs defending the faith.
http://www.usccb.org/bible/2mc/12
I recognize it is not in your protestant canon, but again, the Pope you spoke of was speaking as a Catholic and using Catholic Theology based on the Canon of the Catholic Church.
Thus, I understand that you might not accept this, but it is not as the Pope was pulling this out of thin air. There was and is a sound theological basis for what he was saying with respect to Soldiers who legitimately were fighting to defend the Faith versus those who were fighting to pillage.