Hawzah News Agency ­– If we consider the history of this foundational epoch of Islam, and take into account the regional and global context at the death of the Prophet, the necessity of appointing someone to the position of Imam will be readily apparent. For, in the tumult following upon the Prophet’s death, Islam faced a three-fold threat: on the one side, there was the Byzantine Roman Empire, on another was the Sassanid Persian Empire, and, from within, there was the danger posed by the group known as the ‘Hypocrites’. As regards the first threat, suffice to note that the Prophet was concerned about it right up to his last days, mobilizing a large contingent of Muslims under the command of Usama b. Zayd, to confront the Romans, despite the protests of those who were opposed to such a move. As for the second threat, the Sassanid emperor was clearly
a malicious enemy who, having torn up the letter sent to him by the Prophet, himself wrote a letter to the governor of Yemen instructing him either to capture the Prophet or send to him his severed head. As for the third enemy, these persons had always
been causing trouble for the Prophet, in Medina and elsewhere, their various plots being so many thorns in his side; their machinations and schemes are mentioned in various places in the Qur’an; indeed, an entire Sura—one in which their evil thoughts and actions are commented upon—is named after them.
 

Now it might be asked: in the face of this triple danger, with enemies ready to ambush the Muslims from all sides, would the Holy Prophet have left the religion of Islam and the Muslim community without a leader, leaving the Muslims without any clear guidance?
 

This should also be considered: there is no doubt that the Prophet understood that the life of the Arabs was dominated by tribal loyalty, with members of a tribe seeing their own lives as bound up with that of their chief. Thus, leaving the task of appointing a leader to such tribesmen could only lead to factional disputes and inter-tribal rivalry, allowing the enemies of Islam to take advantage of the divisions opened up in the ranks of the Muslims. It is on this basis, precisely, that Ibn Sina writes: ‘The appointment of a successor by means of explicit designation (nass) by the Prophet is closer to the truth [of the question of the caliphate], for by such designation, every kind of dispute and opposition is uprooted.

Reference:

Ayatollah Jafar Sobhani, Doctrines of Shii Islam, A Compendium of Imami Beliefs and Practices, Translated and Edited by Reza Shah-Kazemi, published by I.B.Tauris Publishers, london • new york  2003.