Contradiction with Genesis 1:31
Joshua 12:21 lists kings defeated, implying conflict is part of creation, while Genesis 1:31 states that everything was "very good," suggesting harmony and no conflict.
Genesis 1:31: And God saw every thing that he had made, and, behold, [it was] very good. And the evening and the morning were the sixth day. [And the evening...: Heb. And the evening was, and the morning was etc.]
Contradiction with Isaiah 2:4
Joshua 12:21 reflects war and conquest, whereas Isaiah 2:4 envisions a time when nations will not train for war and swords are turned into plowshares.
Isaiah 2:4: And he shall judge among the nations, and shall rebuke many people: and they shall beat their swords into plowshares, and their spears into pruninghooks: nation shall not lift up sword against nation, neither shall they learn war any more. [pruninghooks: or, scythes]
Contradiction with Matthew 5:9
Joshua 12:21 involves warfare, but Matthew 5:9 praises peacemakers, suggesting a preference for peace over conflict.
Matthew 5:9: Blessed [are] the peacemakers: for they shall be called the children of God.
Contradiction with Romans 12:17-18
Joshua 12:21 involves conquest and battle, while Romans 12:17-18 instructs believers to live peaceably with all men and not repay evil for evil.
Romans 12:17-18: Recompense to no man evil for evil. Provide things honest in the sight of all men.