عَنِ ابْنِ عَبَّاسٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَوْمَ الْفَتْحِ فَتْحِ مَكَّةَ " لاَ هِجْرَةَ وَلَكِنْ جِهَادٌ وَنِيَّةٌ وَإِذَا اسْتُنْفِرْتُمْ فَانْفِرُوا "
Bukhari: God's Apostle said on the day of the conquest of Mecca, "There is no migration now, only Jihad and good intentions. And when you are called for Jihad, you should come out at once." (Volume 4, Book 53, Number 412)
The reason is very clear why the Prophet Muhammad (peace be upon him) has mentioned this. When the Prophet Muhammad (peace be upon him) entered Makkah, the city of those who had persecuted him and his followers a decade before, he forgave those who persecuted him. It was this mercy that allowed the entire city to willingly embrace Islam. Muhammad Husayn Haykal writes about the Prophet's entry into Makkah:
The Prophet camped on a height opposite the mountain of Hind and in the proximity of the graves of Abu Talib and Khadijah. He was asked whether he wanted to rest in his old house in Makkah and answered, "No! They have leveled it." The Prophet then retired to his tent grateful to God for this glorious and victorious return, and for bringing to its knees the cruel city which had tortured and banished him. For a moment, he turned his gaze toward the valley of Makkah as well as to the surrounding hills. He recalled that in those hills he often found refuge from the persecution of Quraysh... ...Mounting his she camel, al Qaswa', he rode toward the Ka'bah where he circumambulated the House without dismounting. He then dismounted and called upon `Uthman ibn Talhah to open the Ka'bah for him. Muhammad stood at the door surrounded by the many worshippers who had found their way to the holy House. He delivered a speech to the people present in which he said, quoting the Qur'an... ..."O men, We have created you from male and female and constituted you into peoples and tribes that you might know and cooperate with one another. In the eye of God, highest among you is the most virtuous. God is omniscient and all wise." [Qur'an, 49:13] He continued: "0 Men of Quraysh, what do you think I am about to do with you?" "Everything good," they answered, "for you are a noble brother and a noble nephew of ours." Muhammad went on: "Rise, then, and go. For you are free." With this word, Muhammad gave a general amnesty to all Quraysh and all the Makkans. The Prophet's General Amnesty Oh, the beauty of pardon and forgiveness on the part of the mighty and powerful! How great is the soul of Muhammad which rose above hatred and above revenge, which denied every human feeling and ascended to heights of nobility man had never reached before! There were the Quraysh among whom were people whom Muhammad well knew had plotted to kill him, had persecuted him, and inflicted upon him and his companions all kinds of injury and harm, who fought him at Badr and at Uhud, who blockaded him in the Campaign of al Khandaq, who incited the Arab tribes to rise against him, and who would even then tear him apart if only they had the power. There, the whole of Quraysh stood totally under Muhammad's hand, indeed under his feet, totally subject to his command. Indeed, their very life depended upon the first word emerging from his lips. All these thousands of men, of Muslims in battle array, stood on the ready waiting for that one word to wipe out the whole of Makkah and its people within minutes. Muhammad, however, was no less than Muhammad! He was no less than the Prophet of God! No alienation, antagonism, or hostility could find any permanent abode in his heart. His heart was absolutely free of injustice, of malice, of tyranny or false pride. In the most decisive moment, God gave him power over his enemy. But Muhammad chose to forgive, thereby giving to all mankind and all the generations the most perfect example of goodness, of truthfulness, of nobility and magnanimity. (Haykal, The Life of Muhammad ]; US North American Trust Publications, 1976, pp. 406-408)
At this point, Islam had become the major authority in the Arabian Peninsula and was established as a full state. With this many followers it was clear that in the face of danger, migration was no longer an option for such a large nation as no Muslim could turn their back on a fellow believer. Rather it was imperative that they all recognize the need for standing united in defending against the inevitable attacks from the armies of Persians and Romans.