Well.. Christianity is not a teaching of this world.. to say it in that way.. (as some others have mentioned already)
I believe that the events of the OT was an image of what would happen in the NT and later (today). The events in the OT was concrete, physical.. the corresponding events in the NT> are spiritual..
Israel waged physical war - Christians wage spritual war (by praying, witnessing about Jesus to others etc.)
Israel wandered around in the desert for 40 years, struggling phyically - Jesus was tempted by Satan in the desert for 40 days, spiritually (though also physically tested).
Paul states to us in Ephesians 6, that our war is not on flesh and blood, but against the evil spirits. Contrary; Israel fought numerous nations, and it happened according to the will of God (God even got mad at Israel when they disobeyed Him, and didn't completely wipe out certain peoples)
OT was the first covenant.. but as we see in the Hebrewes (NT), Jesus instated a newer, and better convenant, the New Covenant, that we now live under.
So it's quite understanable that you question this.. I have also thought about it many times.. but after a while, and after reading more about it (in the Bible) I could understand more and more.. (I'm not saying that I understand it fully, but I know more today than before)
Good luck in your search for understanding this better. God bless you Bro