A large army was raised in the fifth year of the Hijra. The strength of this army has been estimated by historians as between ten and twenty-four thousand men. But a confederated army raised out of the different tribes of Arabia could not be an army of ten thousand. Twenty-four thousand seems nearer the truth. It could easily have been eighteen or twenty thousand. The town of Medina which this horde wished to attack was a modest one, quite unable to resist a concerted attack by all Arabia. Its population at this time was little more than three thousand males (including old men, young men and children). Against this population the enemy had raised an army of twenty to twenty-four thousand able-bodied men, experienced in warfare; and (having been assembled from different parts of the country) they were an army with a well-selected personnel. The population of Medina, on the other hand, which could be called upon to resist this huge army included males of all ages. One can judge the odds against which the Muslim population of Medina had to contend. It was a most unequal encounter. The enemy was twenty to twenty-four thousand strong, and Muslims hardly three thousand including, as we have said, all the males of the town, the old and the young. When the Prophet(sa) heard of the huge enemy preparations, he held a council and asked for advice. Among those who were consulted was Salman(ra) the Persian, being the first Muslim convert from Persia. The Prophet(sa) asked Salman(ra) what they did in Persia if they had to defend a town against a huge army. "If a town is unfortified, and the home force very small," said Salman(ra), "the custom in our country is to dig a ditch round the town and to defend from inside." The Prophet(sa) approved of the idea. Medina has hills on one side. These provided a natural protection on that side. Another side with a concentration of lanes had a compact population. On this side the town could not be attacked unawares. The third side had houses and palm-groves and, at some distance, the fortresses of the Jewish tribe, Banu Quraiza. The Banu Quraiza had signed a pact of peace with the Muslims. Therefore this side was also considered safe from enemy attack. The fourth side was an open plain and it was from this side that the enemy attack was most likely and most feared. The Prophet(sa), therefore, decided to dig a ditch on this open side so as to prevent the enemy from attacking unawares. The task was shared among Muslims—ten men were to dig ten yards of the ditch. Altogether a mile long ditch, of sufficient width and depth, had to be dug.
When the digging was going on, they came upon a rock which Muslim sappers found hard to tackle. A report was sent to the Prophet(sa) who made for the spot at once. Taking a pickaxe he struck the rock hard. Sparks came out and the Prophet(sa) cried aloud "Allahu Akbar". He struck again. Again a light came out and again the Prophet(sa) cried out, "Allahu Akbar". He struck a third time. Light came out again, the Prophet(sa) said, "Allahu Akbar" and the rock was in fragments. The Companions asked the Prophet(sa) about all this. Why did he say, "Allahu Akbar" again and again?
"I struck this rock three times with this pickaxe, and three times did I see scenes of the future glory of Islam revealed to me. In the first sparks I saw the Syrian palaces of the Roman Empire. I had the keys of those palaces given to me. The second time I saw the illumined palaces of Persia at Mada’in, and had the keys of the Persian Empire given to me. The third time, I saw the gates of San’a and I had the keys of the Kingdom of Yemen given to me. These are the promises of God and I trust you will put reliance in them. The enemy can do you no harm" (Zurqani, Vol. 2).
With their limited man-power, the ditch which the Muslims were able to dig could not be a perfect one from the point of view of military strategy, but it at least seemed to ensure against the sudden entry of the enemy into the town. That it was not impassable, subsequent events in the battle amply proved. No other side suited the enemy from which to attack the town.
From the side of the ditch, therefore, the huge army of Arabian tribesmen began to approach Medina. As soon as the Prophet(sa) got to know of this, he came out to defend it with twelve hundred men, having posted other men to defend other parts of the town.
Historians estimate differently the number which defended the ditch. Some put it at three thousand, others at twelve to thirteen hundred, still others at seven hundred. These estimates are very difficult and apparently difficult to reconcile. But, after weighing the evidence, we have come to the conclusion that all the three estimates of the Muslim numbers engaged in defending the ditch are correct. They relate to different stages of the battle.