Complex sentence: On 15--21 March, his 2nd Division of the V. Corps besieged and captured the small fortified town of Campo Maior, in eastern Portugal, during which the French captured 50 guns and the 100-man Portuguese garrison.
Split into simpler sentences: On 15--21 March, his 2nd Division of the V. Corps besieged and captured the small fortified town of Campo Maior, in eastern Portugal. There they captured 50 guns and the 100-man Portuguese garrison.