Complex sentence: Under Duke Jacob Kettler (ruled 1642-1682), a Baltic German, it established one of the largest merchant fleets in Europe, with its main harbours in Windau (today's Ventspils), and Libau (today's Liepāja).
Split into simpler sentences: Under Duke Jacob Kettler, a Baltic German, it established one of the largest merchant fleets in Europe. Kettler established one of the largest merchant fleets in Europe, with its main harbours in Windau (today's Ventspils), and Libau (today's Liepāja).