Why did the Ansar surround Medina with swords when the Prophet (saw) arrived there?

In a Nutshell: The event reflected the establishment of the first Muslim state; the swords drawn by the warriors was to let the people of Medina know there was new political leadership. Historical Background When the Prophet (saw) and his companions (ra) were persecuted in Mecca, the Prophet sought the protection and support of tribal leaders. He asked them to protect him and Islam against the enemies of Islam. Later the Ansar accepted the invitation and pledged protection and support and asked the Prophet (saw) to migrate to lead their state. When the Prophet (saw) migrated, the Ansar did two different things: fifty men surrounded him by swords and every clan requested the Prophet (saw) to live among them saying, 'come to the (largest) number, (best) equipped and protection.' Ibn Kathir narrated: ْوكَانَ أولُ مَنْ بَصُرَ بِهِ رجلٌ مِنَ اليَهُودِ … فَنَادَى بِأَعلى صَوتِهِ: يا بني قِيلَة هَذَا جَدُكُمُ الذي تَنْتَظِرُون! فَخَرَجَ الأَنْصَارُ في سِلَاحِهِم. "The first one to see the Prophet (saw) (at the doors of Medina) was a Jewish man who loudly shouted, 'O Bani Qeelah (Ansar) this is your forefather whom you are waiting for.' Ansar went out armed with their weapons." (Ibn Kathir, al-Fusul fi Seerat al-Rasul, p. 117) Ibn Hiban added: فَثَارَ المُسْلِمُونَ إِلَى السِّلاَحِ فَتَلّقُوا رَسُولَ اللهِ صَلَّى اللهُ عَلَيهِ وَسَلََّمَ بِظَهْرٍ الحَرَةِ وَهُمْ خَمْسُمِائَةِ رَجُلٍ مِنَ الْأَنْصَارِ "Muslims surged to their weapons and met the Messenger (saw) at Harrah and they were five hundred men of Ansar." (Ibn Huban, as-Seera al-Nabawiyah wa Akhbar al-Khulafa', Vol. 1, 139) Once the Prophet (saw) entered Medina, the clans started pulling the bridle his camel asking him to live in their home. They used to say, هَلُمَّ إلَيْنَا، إلَى الْعَدَدِ وَالْعِدَّةِ وَالْمَنَعَةِ "Come to the (large) number, (best) equipment and protection." (Ibn Hisham, as-Seera al-Nabawiyah, Vol. 1, p. 394) But the Prophet (saw) would excuse them saying, خَلُّوا سَبِيلَهَا فَإِنَّهَا مَأْمُورَةٌ "Leave (the camel) because it is ordered (by Allah to sit in a certain place)." (Ibid) What do these events mean? When we look at this historical context, the Prophet (saw) was seeking a polity to adopt the Islamic worldview to further the dawa; the pledge of the Ansar to fight the enemies of Islam regardless of their number, equipment and kin; the migration of the Prophet (saw) to lead the new state with them all pledging to obey him. They surrounded the Prophet (saw) by swords as a sign of the establishment of the first Muslim polity, where the army, weapons, leader, laws and protection were fir Islam. Ibn Kathir commented on this saying: َولَمَا اسْتَقَرَّ رَسُولُ اللهِ صَلَى اللهُ عَليْهِ وسَلَّمَ بِالمَدِينَةِ بَيْنَ أَظْهُرِ الأَنْصَارِ وتَكَفَّلُوا بِنَصْرِهِ ومَنْعِهِ منَ الأَسْودِ والأَحْمَرِ "When the Prophet (saw) stayed in Medina among the Ansar, they guaranteed his protection and support against the black and white (of his enemies)." ((Ibn Kathir, al-Fusul fi Seerat al-Rasul, p. 121) What supports this understanding is the reaction of one of the leaders of Medina i.e. Abu Amir. He rejected Islam believing his kingdom was stolen from him. When the Prophet (saw) entered Medina, he fled to Mecca to the enemies of the Prophet with a number of soldiers. There he demanded preparation for war to expel the Prophet (saw), to end up the new Muslim state gain back his state. When the Prophet conquered Mecca, he fled to Ta'if; when Ta'if accepted Islam, he fled to Syria and died there. Ya'muri narrated: وَأَمَّا أَبُو عَامِرٍ فَأَبَى إِلَّا الْكُفْرَ وَالْفِرَاقَ لِقَوْمِهِ حِينَ اجْتَمَعُوا عَلَى الإِسْلامِ، فَخَرَجَ مِنْهُمْ إِلَى مَكَّةَ بِبِضْعَةَ عَشَرَ رَجُلا مُفَارِقًا لِلإِسْلامِ "But Abu Amir accepted none but disbelief and leaving his people when they collectively accepted Islam. He went out to Mecca with more than ten men leaving Islam." (Ya'muri, 'Uyun al-Athar, Vol. 1, p. 225) Ya'muri said leaving Islam, where in reality he left Medina; but he meant Medina was the manifestation of Islam. Conclusion The event reflected the establishment of the first Muslim state; to let the people of Medina know there was new political leadership. Main References Ibn Kathir, al-Fusul fi Seerat al-Rasul Ibn Huban, as-Seera al-Nabawiyah wa Akhbar al-Khulafa' Ibn Hisham, as-Seera al-Nabawiyah Ya'muri, 'Uyun al-Athar