Complex sentence: On 14 September the column was tasked with storming the Kashmir Gate--a part of the walled defences of Delhi--which was successfully achieved, but fierce fighting ensued and Delhi was not fully retaken until 20 September.
Split into simpler sentences: On 14 September the column was tasked with storming the Kashmir Gate--a part of the walled defences of Delhi--which they successfully achieve. Fierce fighting, however, ensued and Delhi was not fully retaken until 20 September.