Who could they fight?
Christians miss the fact in scripture that Old Testament Saints were only equipped to fight against people, they were not equipped to fight against their true enemy, the forces of darkness.
So that's what was recorded, and influenced their beliefs, they didn't see the fingerprints of darkness influencing people so they fought only who they were equipped to fight.
God in covenant with the Israelites protected them from destruction when He could.
Their enemies who had been seduced by darkness and quashed and pushed away any amount of God's covering by their flirtation with sin, were led into battles they could not win by satan.
satan, who despises God's favorite creation, man, also was successful at times in seducing the Israelites. At times he would seduce them into sin and quash or remove God's covering, and would lead them to destruction as well.
In the NT Jesus reveals who our true enemy is, the forces of darkness, He then equips us to fight a spiritual war. He goes to great lengths to show that people are not our enemy, satan and his works are our true enemy.
The problem that comes from this bad belief that we are at war with "people", and this overlooking of truth in scripture, is that the church has now engaged in a culture war for centuries fighting people.
Fighting against the ones they are to be guiding free from darkness' hold.
Christians are too busy fighting other people rather than fighting the forces of darkness, so on their watch darkness is running rampant and instead of guiding people home, they are pushing people into satan's arms.
The atheist is not your enemy, neither is the homosexual, the muslim, or a wide variety of "people" the church calls their enemies.
The true enemy, yet again, is darkness.
Darkness that steals, kills, and destroys.
Darkness that plants perverse hateful thoughts and desires into people.
Darkness that seduces man into believing a lie of themselves and guides them to sin.
The church would be a lot more successful if they spent less time idolizing man, and traditions of man, and simply listened to Jesus.