"Although all Israel was assembled together against [the Benjamites in Judges 20] without any dissension or rebellion as though they were all one man, yet they never obtained the victory against their enemies until they humbled themselves before God by fastings and prayers and asked pardon for their sins and placed their pride and their glory in the dust (Judg. 20:26).

"And then, when they had wholly rejected their own power and might and had placed all their trust in God instead of in their great multitude and in their horses and weapons and gave Him all the glory, they learned by experience that He is not without good reason called the Lord of Hosts. For, though they had a righteous cause, and though God was with them and they had every advantage—even according to worldly standards—yet God desired to humble them. For He resists the proud but gives grace to the humble (1 Pet. 5:5; Jas. 4:6).

Now the Israelites were so prideful before they humbled themselves that it seemed to them that they could swallow their enemies whole and do whatever they wanted with them.

Therefore God willed to show them that victory lies neither in force of
arms nor in the multitude of men,

nor even in a righteous cause,

but in Him alone. For we can indeed take a cause which is righteous before God and make it a wicked one by our guilt and sins and by what we add of ourselves."

Pierre Viret (1510 – 1571), Swiss Reformed theologian, evangelist and Protestant reformer