Fear not, neither be thou dismayed:

And the LORD said unto Joshua, Fear not, neither be thou dismayed: take all the people of war with thee, and arise, go up to Ai: see, I have given into thy hand the king of Ai, and his people, and his city, and his land:

On many occasions in our Christian lives after we sin we feel so defeated that we can't go forward, however in the case of Israel from the previous post they had repented of their sin. We see that it was the nation of Israel that allowed sin to enter the camp, but it was the sin of one person that ruined it for all. Achan had been defeated by his flesh, and with the pride of Israel by not sending their whole army forward they fled before the men of Ai. Well now things are different they had cleansed the camp in the sight of the Lord, and He now says to go defeat Ai, that is in representation the flesh. If you'll notice after they had repented God did not bring that before their face, it was forgiven, they could go forward, no looking at the ground and sulking in defeat. We see here that they sent all the army out this time, because folks we have to bring all that we got against the flesh. We cannot just rely on ourselves to defeat this awful flesh, no we must bring God with us into the battle. We see that once Israel defeated Ai, they read the whole law, not just a part, they read it all to remind them of what they were to be doing. We should also take this as an example and daily be reading the Bible, and spend time in prayer. There's only one way to defeat the flesh and that's to stay humble and walking with Christ in an ever growing relationship. If for some reason you say Brother all this happened in the old testament, don't be fooled by such an admonition for here is Paul warning us in the new...
Rom 8:13 For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live. Put away those things which you ought not be doing and trust the Lord Jesus Christ to help you to wash away your sins and never bring them before you.