In 627, the eighteenth year since Muhammad’s move to Medina, the Quraysh of Mecca gathered allies to make a full assault on the city of Medina. The first ally to volunteer was the banished Jewish tribe of Banu Nadir. They were now living about 100 miles north of Medina in the walled town of Khaybar. The Banu Nadir actively sought out more confederates for the army, paying a nomadic tribe called the Banu Ghatafan half their harvest to come and join. Some other nomadic tribes joined, including the Kinanah who had emigrated from Yemen at the time of the Marib Dam’s failure. The area around Medina from Khaybar in the north, and along the east where the nomads were, to Mecca in the south, was allied against the Muslims.
As many as 10,000 men with 600 horses marched from several directions to converge on Medina. Friendly nomads warned Muhammad just in time, so that he knew the army was coming with only a week to spare. The Muslims had to decide: given a week, what could they do? Choose a field, and meet them there? Send for their own allied nomads, but could they come in time? Stay in the city, to defend it?
One of the men now living in Medina was an unusual foreigner: Salman al-Farisi was a Persian (which is what al-Farisi means), a former Zoroastrian fire priest. He had traveled widely and lived a number of years as a slave in Medina. He had practiced Islam by himself, as a slave, with only sporadic contact with the other Muslims. At last, Salman had asked Muhammad to help him buy his freedom. He was now fully part of the Muslim community, and they must have recognized that he had linguistic and scientific knowledge from outside their scope.
In this case, Salman proposed that they stay in the city but create a barrier to prevent an effective cavalry charge. Persians, he said, would dig a trench. Since Medina was surrounded by rocky hills and the old lava beds, there were well-defined spaces that would need to be trenched. It was just possible that, given a week and all hands to work, they could get it done.
Salman had worked for the remaining large Jewish tribe, the Banu Qurayzah, and he knew that they owned some excavating equipment. The Muslims borrowed these tools and set everyone to work. The planned trench would seal off the open end of the combination of hills, trees, and lava that surrounded them. It would be over five kilometers long, stretching from hill to hill. It had to be wide enough and deep enough to slow down horses, putting them at a disadvantage to archers and spearmen as they struggled up the other side. One estimate is that it was about 30 feet wide, and 10 to 15 feet deep. Muhammad came out to help dig, and he led the men in singing songs of faith as they worked. Boys helped carry away the dirt in baskets.
There is one story of the digging that is not believable to non-Muslims, but it’s worth telling because it shows the beginning of the idea to unite the Arabian peninsula (and beyond). Down in the ditch, there was a huge boulder that could not be moved or split. Muhammad struck it with three blows, and it split into movable pieces with flashes of light. In these flashes of light, he saw visions that Allah would soon give the Muslims control over the south (Yemen), the north (Syria), and even the east (Persia).
When the Meccan armies arrived, the trench was nearly finished; one place was still only a meter or two wide. The outlying villages had evacuated, taking their supplies into the city, and women and children had been sent into upper rooms of the fortresses. Muhammad and the 3000 fighting men—a few lucky teenage boys had been granted permission to join them—camped across from the trench, ready to defend its length. They pitched tents and built fires.
The Meccans had not expected anything like the trench. They moved up and down it, inspecting it and thinking. One of the Jews from Banu Nadir (i.e. he was an ex-Medina resident) offered to slip inside the city and speak to the Jewish tribe that had lent the Muslims their digging tools. He thought he could persuade them to turn on the Muslims. So he did it, and after some resistance, the leader of the Banu Qurayzah Jews agreed. He ripped up the parchment treaty with Muhammad. What seems to have persuaded him was the sheer size of the Meccan force. They wanted to be on the surviving side, obviously, and it didn’t look like it would be Muhammad’s side. He promised that the Banu Qurayzah would attack Medina from within the defenses. This way the Muslims would need to guard and fight on two fronts, so the trench defense would be weakened.
Then the war just fizzled. The trench was such an effective deterrent that no more than an occasional single horse could get through. The Banu Qurayzah knew that enough men had come back to Medina to patrol the streets that any attack would be a bloodbath, so they never got around to actually making their internal attack. It’s notable in these battles that Arabians seemed averse to bloodbaths, cf. the Meccans after Uhud being deterred by a force one-tenth their size. Arabians liked surprise attacks, quick raids, and overwhelming numbers.
As time went by, Muhammad negotiated with one of the nomadic tribes, offering dates for them to leave, and there were showers of arrows now and then. Then one of the nomad chiefs suddenly quit; he quietly went into Medina’s camp to declare that he was a Muslim now. Muhammad asked him to return just as quietly, as though nothing had happened, and see if he could get the confederation to break apart. So the Bedouin chief went first to the Jews of Medina (the Banu Qurayzah) and suggested to them that the Quraysh planned to sell them out with a separate peace. Then he told something similar about the Jews to the Quraysh.
Additionally, time was passing. The nomads had come to make a quick assault, not to sit around campfires for two weeks. Finally, a fierce storm fell on the valley, with torrents of rain and hurricane-strength wind. Toward dawn, the Meccan leader Abu Sufyan called for an end to the expedition and retreat to their homes.
After the dawn prayer, the Muslims could see the abandoned camp, and they too returned home. There really had been no battle at the Battle of the Trench, and it was over.