Complex sentence: He had kept them behind his infantry line, intending to allow the buccaneers to pass through his lines, and setting them against the attackers to presumably disrupt and disorganize them just before the Spanish foot made contact with the buccaneering force.
Split into simpler sentences: He had kept them behind his infantry line, intending to allow the buccaneers to pass through his lines, and setting them against the attackers. It was assumed this would have disrupted and disorganized their forces just before the Spanish foot made contact with the buccaneering force.