In A.D. 630 Meccans attacked a tribe which was under Muslim protection, and thus broke the truce. The prophet PBUH led a large army of Muslims (10000 strong) towards Mecca. By now the tide had turned; the Meccans were not strong enough to fight such an army commanded by a great general such as the Prophet PBUH turned out to be, and they laid down their arms. Their leader, Abu Sufyan, accepted Islam. His son, Muawiyah, became one of the secretaries of the Prophet PBUH. Mecca, the heart of Arabia, now lay before the Prophet PBUH. The despised son of that ancient town had come back triumphant, but instead of being vindictive, he showed kindness and mercy toward his arch enemies. He acted in a way new to the Arabs; he forgave his enemies. He destroyed the idols which lay in the Kaaba, but the first thing he did in the Kaaba was to pray.
He gave back the key of the sacred house to the hereditary keepers and told the people to come in peace to him. This conduct delighted the Meccans. They considered this great act of kindness and forgiveness of the Prophet PBUH the height of nobility and came forward to enter this astonishing religion which returned enmity with love.
The conquest of Mecca is great event in the history of Islam because after this conquest Islam spread in all corners of the world beyond Arabia. Prophet Muhammad PBUH won that great battle without any bloodshed.