Complex sentence: The squadron under Commodore Thomas Truxtun was patrolling the waters between Puerto Rico and Saint Kitts when his flagship "Constellation", cruising independently, met and engaged "L'Insurgente".
Split into simpler sentences: The squadron under Commodore Thomas Truxtun had been sent to patrol the waters between Puerto Rico and Saint Kitts. Truxtun's flagship "Constellation" was cruising independently when he met and engaged "L'Insurgente".