Complex sentence: In a conference they decided on a combined attack against Nevis and were further strengthened by 600 volunteers raised on Martinique plus another 500 picked up two days later at Guadeloupe.
Split into simpler sentences: In a conference they decided on a combined attack against Nevis. They were further strengthened by 600 volunteers raised on Martinique plus another 500 picked up two days later at Guadeloupe.