Contradiction with Exodus 20:13
Joshua 8:1 involves God commanding Joshua to take military action, while Exodus 20:13 prohibits killing.
Exodus 20:13: Thou shalt not kill.
Contradiction with Matthew 5:44
Joshua 8:1 involves preparing for conflict, whereas Matthew 5:44 teaches love for enemies and prayer for those who persecute.
Matthew 5:44: But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
Contradiction with Romans 12:19
Joshua 8:1 suggests taking vengeance on Ai, while Romans 12:19 advises leaving vengeance to God.
Romans 12:19: Dearly beloved, avenge not yourselves, but [rather] give place unto wrath: for it is written, Vengeance [is] mine; I will repay, saith the Lord.
Contradiction with Isaiah 2:4
Joshua 8:1 involves warfare, whereas Isaiah 2:4 speaks of turning swords into plowshares and not learning war anymore.
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 26:52
Joshua 8:1 involves using the sword for conquering, while Matthew 26:52 warns against living by the sword.
Matthew 26:52: Thinkest thou that I cannot now pray to my Father, and he shall presently give me more than twelve legions of angels?