Proverbs 21:31 is in focus today, though you would do well to also read Proverbs 21:30 alongside it. These two verses point out the frustration of attempting without the empowerment of God Almighty.
How so? The horse is prepared for battle—consider this reality: horses on a field of foot soldiers are a big advantage. Big. The elevation of sight perspective, the angle of attack, all of these cause horses to be very useful. Even an infantry unit, pre-mechanization, was often commanded by leaders on horseback.
Having horses prepared was a crucial part of battle readiness. The more horses, the greater the advantage (this is simplifying, but still valid.) One should not go to battle without horses prepared.
The problem is this: even with all the horses in the world, God can turn a battle in a different direction. Victory is from YHWH, not from horses. This applies in a couple of ways:
First, being outnumbered is no major issue if God is on your side. This does not counter using proper wisdom in preparation—after all, the Proverb is not “don’t bother with the horse, victory is from YHWH.” However, victory is born from obedience rather than tactical advantage.
Second, being in the power position is not where credit belongs for victory. Perhaps you did prepare the horse, and you had more than the other guy. You won—but that victory was granted by God through your obedience, not achieved by you without His involvement.
The street runs both directions: some overcome terrible odds in life because God is with them. Others succeed with the odds in their favor because God has granted them those odds.
Either way, it is our responsibility to prepare our horses—and to let God bring victory to those who obey Him!