Some argue that the Bible is contradictory concerning whether God is a God of peace or war. The argument is that Exodus 15:3 and Joel 3:9-10 say that God is a God of war, but Isaiah 2:4 and Romans 15:33 say that God is a God of peace. However, there is a very simple explanation that solves this alleged contradiction.
The Lord is a man of war; the Lord is his name.Exodus 15:3
9 Proclaim this among the nations: Consecrate for war; stir up the mighty men. Let all the men of war draw near; let them come up. 10 Beat your plowshares into swords, and your pruning hooks into spears; let the weak say, “I am a warrior.”Joel 3:9-10
He shall judge between the nations, and shall decide disputes for many peoples; and they shall beat their swords into plowshares, and their spears into pruning hooks; nation shall not lift up sword against nation, neither shall they learn war anymore.Isaiah 2:4
May the God of peace be with you all. Amen.Romans 15:33
God is a God of war in the sense that he will conquer all of his enemies. God is a God of peace in the sense that he grants peace to his people. In other words, God is a God of war towards his enemies, or those who do not submit to him, and God is a God of peace towards his people, or those who submit to him. There is simply no contradiction between these two concepts.
There are some foundational principles that apply to all alleged and apparent contradictions in the Bible. To read more, see Bible Difficulties: Foundational Principles.
To read more answers to alleged and apparent contradictions in the Bible, see “Contradictions” in the Bible Answered.
These books are also excellent resources: