Assuredly Allah did help you in many battle-fields and on the day of Hunain: Behold! your great numbers elated you, but they availed you naught: the land, for all that it is wide, did constrain you, and ye turned back in retreat. At-Tawbah 9:25 ⧉
Tafsir:
Reliance on oneself develops sort of a haughtiness in a man which results in carelessness of external facts. He falls short in carrying out disciplinary regulations. Due to overweening self-confidence, he starts acting in an unrealistic manner, the result of which is necessarily defeat in this world of cause and effect. As opposed to this, having confidence in God is reliance on the greatest power. This develops humility in a man; he becomes extremely realistic; and realism is the key to all success.