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Exodus 15:3 The LORD is a man of war: the LORD is his name.
Romans 15:33 Now the God of peace be with you all. Amen.
Let's ask a simple question -- is it not reasonable to say that God can be both a "warrior" (NIV) and a God of peace, as the circumstances require? Can a person not be a soldier at some time in their life, during a time of war, and say, a tuba inspector during times of peace? Is God not allowed to react to what we do as He sees fit?
The Exodus was a time when God advanced on behalf of Israel for conquest -- it is certainly appropriate that this hymn of praise to Him extolled that virtue and practice. Romans 15:33 is the salutation of a letter, when blessings are offered. The "genre gap" here is vast, to say nothing of the historical contexts! A contradiction would only exist if God were described as being of war and peace at the same time and performing the same action. That simply isn't the case here. Being warlike and peaceful are not permanently, mutually contradictory states, for they each involve complexes of interaction with persons and actions that are encountered over time.