How could the tiny Muslim garrison prevent the Makkan army...

How could the tiny Muslim garrison prevent the Makkan army from overrunning Medina which would be overwhelmed by sheer numbers, was a question on everyone's mind. One of the closest friends of Muhammad, the Messenger of God, was Salman the Persian. He was born and brought up in Persia (Iran) but had spent many years in Syria and Palestine, and he had familiarity with the warfare and the siege operations of both the Persians and the Romans.

Medina had natural or man-made defenses on three sides but was exposed on one, i.e., the north side. Salman told the Prophet that if a trench were dug on the north side, the city could perhaps be defended successfully. The idea, though new and unconventional in Arabia, appealed to the Prophet. He accepted it and ordered Muslims to dig the trench.

Muhammad Husayn Haykal Salman al-Farsi, who knew far more of the techniques of warfare than was common in the Peninsula, advised the digging of a dry moat around Medina and the fortifications of its buildings within. The Muslims hurried to implement this counsel. The moat was dug and the Prophet – may God's peace and blessings be upon him – worked with his hands alongside his companions lifting the dirt, encouraging the Muslim workers, and exhorting everyone to multiply his effort.

(The Life of Muhammad, Cairo, 1935) Since the Makkan army was known to be approaching Medina rapidly, there was no time to lose, and the Muslims worked frantically – in relays. In six days the trench was dug, just in time to prevent the invaders from taking the town by assault. The Makkan cavalry came like a whirlwind but was suddenly checked, in its career, by the trench. The horsemen reined in their horses at its edge.

Their grand strategy had been to take Medina by storm in a few hours but now it appeared to them that they could not do so. Here there was a trench, a new obstacle which they could not surmount. How did it fit into their strategy? They were utterly nonplused by the trench. Eventually, and after long deliberation, the Makkan commanders decided to lay siege to Medina, and to force the Muslims to surrender, through attrition. They sealed all exits from Medina, and hemmed in the Muslims.

Medina was in a state of siege! Though it was Abu Sufyan who had organized the whole campaign, and he was its director of operations, he was no fighting man himself. The fighting man of his army was Amr ibn Abd Wudd, the fiercest of the warriors of pagan Arabia.