Complex sentence: After a fierce skirmish on 14 October, the French dug themselves in rather than launch a full-scale assault, remaining entrenched for a month before withdrawing to a position between Santarém and Rio Maior.
Split into simpler sentences: After a fierce skirmish on 14 October, the French dug themselves in rather than launch a full-scale assault. They remained entrenched for a month before withdrawing to a position between Santarém and Rio Maior.