Once the main force was lost, the troops fell apart completely.
Tips
history
Attested in 姚雪垠's novel 《李自成》: describing the moment when an army's nerve breaks and it 'scatters so it's no longer an army'. Modern rather than classical in origin.
usage
Strong military imagery. Beyond literal battles, often used of sports teams, debate opponents, competitors — any group utterly beaten. Common pattern: 被 + subject + 打/杀/驳 + 得 + 溃不成军.