Why did the Prophet (saw) start calling other tribes to Islam whilst he was in Mecca?

In a Nutshell: The Messenger (saw) asked his nation to submit to Allah fully, by implementing revelation and helping him convey it to the world. When this failed, he conveyed a message critiquing the fundamental values of the Meccan, traditions of their ancestors and collective social practices. This message was opposed by the tribal leaders of Quraysh and after the death of Abu Talib, the protector of the Messenger (saw), he began seeking the protection and support (mana'a and nusrah) from other tribes to secure his dawa and help convey it to all nations. Preliminary Remarks After the death of Abu Talib, the Meccan leaders started to physically harm the Messenger (saw) and become increasingly hostile to his dawa. As the Messenger (saw) said: مَا نَالَتْ مِنِّي قُرَيْشٌ شَيْئًا أَكرَهُهُ حَتَّى مَاتَ أَبُو طَالِبٍ "Quraysh did not harm me with something I hate except after the death of Abu Talib." (Ibn Hisham, as-Seera al-Nabawiyyah, Vol. 1, p. 417; Tabari, Tarikh al-Tabari, Vol. 2, p. 344; Bayhaqi, Dala'il al-Nubuwwah, Vol. 2, p. 350, Dhahabi, Tarikh al-Islam, Vol. 1, p. 235) At this stage, Allah ordered the Messenger (saw) to start seeking man'ah and nusrah from other tribes: وَقُل رَّبِّ أَدْخِلْنِي مُدْخَلَ صِدْقٍ وَأَخْرِجْنِي مُخْرَجَ صِدْقٍ وَاجْعَل لِّي مِن لَّدُنكَ سُلْطَانًا نَّصِيرًا "And say, "My Lord! Cause me to enter an entrance sound, and cause me to an exit sound and make for me from near You a supportive authority." (Al-Isra 17:80) It was an action based on Allah's order as Ali (ra) said in the context of accompanying the Messenger (saw) in meeting the tribes with Abu Bakr (ra): لَمَّا أَمَرَ اللهُ نَبِيَّهُ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنْ يَعْرِضَ نَفْسَهُ عَلَى قَبَائِلِ الْعَرَبِ "When Allah ordered his Messenger (saw) to present himself to the Arab tribes ..." (Abu Na'eem, Ma'rifat al-Sahabah, 6384) The Messenger (saw) began searching for protection and support from other tribes who would adopt his call; help him delivering it and defending him against any enemy; secure his dawa and implement its teachings i.e. mana'ah. Seeking Protection (mana'a) Shortly after Abu Talib passed away, the Messenger (saw) went first to Ta'if tribe asking them for protection and victory against the enemies of Islam. Ibn Hisham narrated: فَخَرَجَ رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ إلَى الطَّائِفِ، يَلْتَمِسُ النُّصْرَةَ مِنْ ثَقِيفٍ، وَالْمَنَعَةَ بِهِمْ مِنْ قَوْمِهِ، وَرَجَاءَ أَنْ يَقْبَلُوا مِنْهُ مَا جَاءَهُمْ بِهِ مِنْ اللَّهِ عَزَّ وَجَلَّ. "The Messenger (saw) went out to Ta'if seeking the support (nusrah) and protection (mana'a) of Thaqeef (a clan in Ta'if) against his tribe and that they embrace what he received from Allah (swt)." (Ibn Hisham, as-Seera al-Nabawiyyah, Vol.1, p. 419) He initiated discussions with the three leaders of Thaqeef (Abd Yalil, Mas'ud and Habib) who all rejected his call and ordered their followers to stone the Messenger (saw) as he left to return to Mecca. Imam Kinani narrated: فَأَقَامَ بِالطَّائِف لَا يَدَعُ أحدا من أَشْرَافهم إلاّ جَاءَهُ وكلَّمه، فَلم يُجيبوه، وخافوا على أحداثهم "He stayed in Ta'if leaving no one of their leaders except he went to him and spoke with him. They did not accept his Message for fear of (harm might inflict) their youths." (Kinani, al-Mukhtasar al-Kabir, p. 39, Ya'muri, Uyun al-Athar, Vol. 1, p. 155) The Messenger (saw) then asked his uncle al-Abbas (ra) to show him the best tribes that could provide him with support and protection. Al-Abbas (ra) took him to the marketplace where the tribal leaders stayed and explained to him the strengths and weaknesses of various Arabian tribes. Ibn Abbas (ra) narrated: قَالَ لِي رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: " لَا أَرَى لِي عِنْدَكَ وَلَا عِنْدَ أَخِيكَ مَنَعَةً، فَهَلْ أَنْتَ مخرجى إِلَى السُّوق غَدا حَتَّى نقر فِي مَنَازِلَ قَبَائِلِ النَّاسِ " وَكَانَتْ مَجْمَعَ الْعَرَبِ. "The Messenger of Allah (saw) asked me, 'I don't find any protection with neither you nor your brother. So, could you accompany me in the marketplace to quickly see the status of the tribes of people.' (The marketplace) was where the Arabs assemble… (then he presented him the powerful aspects and strengths in each tribe)." (Ibn Kathir, as-Seera al-Nabawiyyah, Vol. 2, p. 159) This narration also demonstrates that the Messenger (saw) was seeking collective protection for his deen, not the mere protection for himself against death or harm, because his uncles played that role while the collective Quraysh tribe rejected him. The Messenger (saw) frequently visited the marketplaces of the Arabs for years asking anyone who could carry him to his people to provide him with support and protection. Ahmed ibn Hanbal narrated he used to shout out in marketplaces: مَنْ يُؤْوِيني؟ مَنْ يَنْصُرُنِي حَتَّى أُبَلِّغَ رِسالَةَ رَبِّي وَلَهُ الْجَنَّةُ "Who would provide me with shelter? Who would provide me with support until I convey the Message of my Lord so that he will be granted paradise?" (Musnad Ahmed Vol. 3, p. 322; Ibn Hajar, Fath al-Bari, Vol. 7, p. 222) The aim of seeking the protection of tribes appears clearly in the following narration, which cites the words shouted to the tribal leaders in the marketplaces: هل من رَجُلٌ يَحْمِلُنِي إلى قَومه، فَإنَّ قُرَيْشًا قَدْ مَنَعُونِي أَنْ أُبَلِّغَ كَلامَ رَبِي "Can any man take me to his people because Quraysh has prevented me from conveying the words of my Lord." (Abu Dawud 3734, Tirmidhi 2934, Ibn Majah 201 and Ahmed Vol. 3, p. 390) In another narration, the Messenger (saw) asked a tribe to provide him with protection and support even if they disliked his message. Asbahani narrated the Messenger (saw) as saying: لا أُكْرِهُ أَحَدًا مِنْكُم عَلَى شَيءٍ، مَنْ رَضِي الذي أَدْعُو إليه قَبِلَهُ، ومَنْ كَرِهَهُ لم أُكْرِهْهُ، وإنَّما أُرِيدُ أَنْ تحْرُزُوني مِمَّا يُرَادُ بِي مِنَ القَتْلِ، فَتَحْرِزُونيِ حتَّى أُبَلِّغَ رِسَالَاتِ رَبِّي عَزَّ وَجَلَّ حتَّى يَقْضِي الله تَبَاركَ وتَعَالىَ لي ولمَنْ صَحِبَنِي بِمَا شَاءَ "I would not compel any of you to accept something; rather whoever likes what I call for, he should accept and whoever dislikes it, I would not compel him (to accept). I want you to protect me in conveying the Messages of my Lord (swt) until Allah grants for me and my followers what he wills." (Asbahani, al-Mustakhraj, Vol. 1, p. 72) Objection by the Tribes One could argue the Messenger (saw) did not accept the invitation of Tufayl al-Dawsi (ra) in providing him with mana'ah and nusrah which nullifies the notion of seeking political power. It was narrated Tufayl (ra) when he embraced Islam he said: يَا رَسُولَ اللَّهِ، هَلْ لَكَ فِي حِصْنٍ حَصِينٍ وَمَنَعَةٍ؟ قَالَ: حِصْنٌ كَانَ لِدَوْسٍ فِي الْجَاهِلِيَّةِ. فَأَبَى ذَلِكَ رَسُولُ اللَّهِ "O Messenger of Allah, do you want a great fortress and man'ah? The fortress of Daws (his tribe) in the Pre-Islamic period. The Messenger (saw) rejected that." (Ibn Kathir, al-Bidayah wa al-Nihayah, p. 247 ) This narration in fact supports the notion, as he would not have asked the Messenger (saw) about it if he was not seeking it. He would have heard his message required power to protect and establish it so it could convey its call globally. As for the Messenger's rejection, I would argue he had not yet been ordered by Allah (swt) to turn away from seeking power from potentially the best and most powerful entity, Quraysh, well respected across Arabia. When Quraysh repeatedly rejected his call, Allah then ordered him to approach the tribes, as the analysis above shows, and he accepted such support. The narration also mentions Allah retained this position for the people of Medina: لِلَّذِي ذَخَرَ اللَّهُ لِلْأَنْصَارِ "For what Allah has kept for the Ansar." (Ibid) In the alternative, one could argue he may have believed they could not provide sufficient protection and support for the deen as they were not tribes frequently engaged in war - the Messenger (saw) was seeking a powerful and respectable entity in Arabia who others would fear as the starting point for his message. Conclusion The Messenger (saw) conveyed a message that critiqued the fundamental polytheistic values of the Arabian societies, traditions of their ancestors and social practices. Such a message was fiercely opposed by the Quraysh. After the death of Abu Talib, the Messenger (saw) began seeking protection and support from other tribes to secure his dawa and convey it to the world. References Ibn Hisham, as-Seera al-Nabawiyyah Tabari, Tarikh at-Tabari Bayhaqi, Dala'il al-Nubuwwah Dhahabi, Tarikh al-Islam Kinani, al-Mukhtasar al-Kabir Ya'muri, Uyun al-Athar Ibn Kathir, as-Seera al-Nabawiyyah Ibn Kathir, al-Bidayah wa al-Nihayah Ibn Hajar, Fath al-Bari Asbahani, al-Mustakhraj