Consider the second generation of Israel, after the Exodus. God would take a people, unskilled in the art of war, into a land that was inhabited with powerful enemies, and drive them out. But weak as Israel was, God would weaken the nation still further: not while they were on the Eastern side of Jordan in relative safety, but after they had crossed over and had encamped right opposite Jericho, God commanded that all the young men in the nation were to be circumcised. And all at one time. Thus all the armies of Israel, weak as they were, were for a time completely incapacitated and left totally exposed to the enemies in the land. With what result? The terror of the Lord gripped the inhabitants of Jericho, and they locked and barred the gates in fear of the miracle-working God of Israel.
Again, consider their war tactics. Priests in white robes and carrying a little box covered with gold, and blowing trumpets... marching about Jericho every day... and on the seventh day going about seven times. Foolishness? But God used this kind of foolishness and weakness to terrify the enemy and to cause the walls of Jericho to fall flat.