Prior to crossing this river, Israel had fought some of the nations of the Negev (Midian, Moab, Amorites).  Now they crossed into Canaan proper to begin a sustained battle against the inhabitants.  Fittingly, their crossing of the Jordan was as miraculous as their departure from Egypt:

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         As soon as the priests who carried the ark reached the Jordan and their feet touched the water's edge, the water from upstream stopped flowing. It piled up in a heap a great distance away (Josh 3:15-16).

        

This miracle is not an embellishment, it is central to the narrative.  The generation going to battle was not the one that walked through the Red Sea.  Only the oldest members, those who were above 40 years, and closer to 45, would have had even a childhood memory of that act of deliverance, and of the meeting with God at Sinai.  For the majority, these were just stories that their parents had taught them.  In crossing the Jordan, God made Himself a Name among this generation, He glorified Himself in their eyes.  Again, this reinforces one of the key themes of Biblical faith -- the interdependence of sacred history and current experience.  It was not enough for this chosen generation, the generation of war, to know that God had been with their parents.  They needed to know that He was with them as they entered a hostile and fortified country.