Complex sentence: Nelson's previously proposed attack on Santa Cruz, in April 1797, had to be aborted when 3,500 troops were redeployed but when Jervis heard that a Spanish treasure fleet was anchored there in July, the potential rewards of a success were too great to ignore.
Split into simpler sentences: Nelson's previously proposed attack on Santa Cruz, in April 1797, had to be aborted when 3,500 troops were redeployed. However, when Jervis heard that a Spanish treasure fleet was anchored there in July, the potential rewards of a success were too great to ignore.