Just today, Turkish President Erdogan announced his intention to seize Manbij and Idlib. The Afrin offensive succeeded largely due to the overwhelming manpower superiority of the Turkish-backed rebels against the YPG, which sought to avoid civilian casualties in the battle. The offensive was relatively short, taking just two months to capture the canton.
Manbij may turn out to be a different story. Many factors make Shahba Canton different from Afrin. Firstly, it is a densely-populated flat plain as opposed to the hilly forests found in Afrin. In other words, it is quite similar to the Al-Bab Plain, which Turkey struggled to wrest under its control from ISIS. Secondly, the US is present in Manbij since it played the key role in liberating the city from ISIS and restore essential services to the population. The US has just announced that it has no intention of leaving. The presence of US military personnel in the area creates a big obstacle for Turkey both politically and militarily. What if Turkey accidentally strikes US forces? The geopolitical consequences would be astounding, leading to a falling off in relations and possibly a US assertion of force. But ignoring the American problem, how would Turkey go about invading Manbij in terms of raw operational planning? Here is a prediction for a likely course of action.
Turkey would decide to attack from the most poorly-defended angle. Look at Afrin: Turkey did not attack along the heavily-fortified southern border of the Canton, where kilometers of trenches awaited rebel forces. Instead they moved on the mountains and across a stream first, seizing the more fortified areas from behind. This is similar to the Syrian Army’s strategy in Eastern Ghouta: capture other areas first, thus negating the need to attack any fortifications head-on.
Where is Manbij most defended? The northern and western axis. The Sajur river provides the SDF with a natural barrier against Turkish advance, and the SDF has also dug a defensive trench on the northern and western side of Manbij city. As such, it is likely that the Turkish Air Force would strike SDF forces near the city to soften the defense, also using TSK artillery.
The primary land move for the Turkish-backed rebels would be to advance south of the SDF-held town of Arima and capture villages eastward, near Abu Mandil and Maqta Hajar Saghir. The rebels would continue to advance eastward, capturing village after village, until they reach the southernmost point of Manbij: Aruda Mountain. This move would completely cut off Manbij from any assistance by the pro-government National Defense Forces militia, as well as making it far harder for civilians to escape the Canton.
Then it would be most prudent for Turkish-backed rebels to move northwards towards Tishrin. This would be somewhat difficult due to the many hills in this area. If rebel forces also crossed the Sajur river to the north and cut off the Qara Qawzak Bridge, then Turkey would have completely cut off Manbij from any outside reinforcement and sealed the fate of the Canton. Then the city would be besieged and Turkish forces would capture the city. The question would be: how hard, if at all, would the SDF fight for the city? In Afrin they did not fight at all, and though Manbij is much better for waging a final battle (as ISIS did in 2016), the SDF would likely want to save civilians from a massacre and surrender the city. For this reason it is also possible that rebel forces leave a corridor along the Bridge or the Dam to allow SDF fighters to escape.
Whatever course of action, it is certain that the Syrian Civil War is nowhere near over, and the Turkish-Kurdish conflict in Syria has just begun.