"If I were to get directly in their rear they would, on finding their retreat cut off, fight so fiercely that I might be overpowered.

Even the most cowardly troops will fight, under those circumstances.
Therefore, while threatening their line of retreat, I still leave it open to them.

It is a maxim in war, you know, always to leave a bridge open for a flying foe." In a few minutes they reached the town.

None had observed their approach, the troops being assembled round the church.

These were at once thrown into confusion, when they found themselves attacked with fury by a large force, of whose existence they had no previous thought.
The Vendeans fought with desperate valour.