Ат-Тавба 9:27

In Ramadan of the year 8 Hijrah, the Muslims successfully subdued the Quraysh and conquered Makkah. This was a virtually bloodless entry into Makkah. But in the very next month of Shawwal of the year 8 Hijrah, they were defeated by the polytheistic tribes of Hawazin and Thaqif, though at the conquest of Makkah, the number of Muslims was only ten thousand, while in the battle against the Hawazin and the Thaqif their number was twelve thousand. The reason for this was that when the Muslims had marched against the Quraysh, they had set out simply relying on God. But while proceeding to fight against the Hawazin and the Thaqif, they were proud of the fact that they were the conquerors of Makkah; that they had an army of twelve thousand men; that now nobody could defeat them. They were successful when they had reposed confidence in God; when they reposed confidence in themselves, they had to face defeat. Reliance on oneself develops a 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.