Contradiction with Matthew 5:39
Joshua 1:12 discusses preparation for battle, while Matthew 5:39 advises against resisting evil with violence or aggression.
Matthew 5:39: But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.
Contradiction with Matthew 26:52
Joshua 1:12 supports military readiness, while Matthew 26:52 suggests that those who use weapons will ultimately suffer by them.
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?
Contradiction with Romans 12:19
Joshua 1:12 talks about taking action for land possession, whereas Romans 12:19 encourages leaving vengeance and judgment 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.