Under his rule, Courland and Semigallia became more independent of its Polish suzerain, reached its peak in wealth, and even engaged in its own overseas colonization, making it one of the smallest, but fastest growing states in the world at that time.

Yet, in the end the results of his rule failed in the confrontation with much stronger powers both directly in the Baltic (Sweden) and overseas (Dutch Republic).

In 1634, he made a grand tour of Europe, visiting Paris, London, and Amsterdam, where he studied shipbuilding.

In 1654, he conquered Tobago with forces from the Das Wappen der Herzogin von Kurland, a double-decker ship armed with 45 cannons, carrying 25 officers, 124 Courlander soldiers, and eighty families of Latvian colonists.

In 1651, the duke contacted Pope Innocent X with an offer of 40 Men -of-wars and 24,000 soldiers and sailors to conquer land in Australia.

After the war ended, he rebuilt the duchy's fleet and retook the island of Tobago from the Dutch.