When the Muslims were defeated in the second round, no one remained with the Prophet except ‘Ali (a.s) and thirteen others of the companions of the Messenger. These thirteen were the first to come back to the Messenger after their flight. It is clear that ‘Ali's defense in that decisive hour was much more valuable than the defense of the thirteen companions put together.
The Messenger … became the target of the pagan's attacks. Whenever a regiment aimed at the Prophet (saww) ‘Ali (a.s) charged the regiment and forced it to retreat.
Thus, we would not be erroneous if we say that ‘Ali (a.s) in this decisive battle had the exclusive honor of being the main defender of the Messenger and his Message, against the forces which no one other than ‘Ali (a.s) could face successfully. The Battle of Badr laid the foundation of the Islamic state, but the Battle of Uhud was about to destroy the foundation, had not a small number of heroes headed by ‘Ali (a.s) been present.
The pagans found that the Battle of Uhud ended in their favor. They defeated the army of the Prophet, and the Muslims lost seventy companions, among them the giant hero: Al-Hamzah, uncle of the Messenger and Lion of God. But the pagan victory was not decisive. Their target was Muhammad (saww) and Muhammad (saww) was still alive. He was the biggest danger to them Therefore, it was necessary for them to have another decisive battle in which they would realize the goal that they could not realize at the Battle of Uhud.
The Battle of Uhud took place during the third year after the Hijrah. Two years later, the third decisive battle in which the pagans gathered their biggest task force, took place.
Muslim in his “Sahih” (Authentic) reported that Sahl Ibn Saad said the following:
The face of the Messenger was cut, and one of his teeth was broken, and the protective dress of his head was broken. Fatima (s.a) daughter of the Messenger, was washing the blood and ‘Ali(a.s) was pouring water he brought by his shield from Al-Mihras. Beholding that the water increased the flow of blood, she burned a mat, put some of its ashes on the wound and the blood stopped.(Muslim, in his Sahih, vol 12 p 148)