Deception In War

Moses
And they will listen to your voice, and you and the elders of Israel shall go to the king of Egypt and say to him, 'The LORD, the God of the Hebrews, has met with us; and now, please let us go a three days' journey into the wilderness, that we may sacrifice to the LORD our God.' (Exodus 3:18 ESV)

And Pharaoh rose up in the night, he and all his servants and all the Egyptians. And there was a great cry in Egypt, for there was not a house where someone was not dead. Then he summoned Moses and Aaron by night and said, "Up, go out from among my people, both you and the people of Israel; and go, serve the Lord, as you have said. Take your flocks and your herds, as you have said, and be gone, and bless me also!" (Exodus 12:30-32 ESV)

Joshua
The Lord said to Joshua, "Don’t be afraid or terrified. Take the entire army with you. Start to go up to Ai. Look! I have given the king of Ai, his people, his city, and his land into your power. Do to Ai and its king what you did to Jericho and its king. But you may take its booty and cattle as plunder. Set your ambush behind the city." (Joshua 8:1-2 CEB)

And Joshua and all Israel made as if they were beaten before them, and fled by the way of the wilderness. (Joshua 8:15 KJV)
Joshua and all Israel let themselves be driven back before them, and they fled toward the wilderness. (Joshua 8:15 NIV)
And Joshua and all Israel pretended to be beaten before them and fled in the direction of the wilderness. (Joshua 8:15 ESV)
Then Joshua and all Israel let themselves be beaten before them. They fled in the direction of the desert. (Joshua 8:15 CEB)

Just as plunder and killing, deception is allowed in war.

Alternate view:
The request to be allowed to make a three-day journey into the desert to offer sacrifices to God was sincere. Why not? Could not the Lord test the king of Egypt first in the lesser thing before He presented him with the demand to release His people? (Exodus – Bible Student’s Commentary, Grand Rapids: Zondervan, 1982, page 57)