When in 1640, Portugal recovered its independence from Spain under John II, 8th Duke of Bragança, the Prince of Nassau-Siegen, anticipating an alliance with the latter, and believing that a treaty of peace with Portugal would leave Holland in possession of the conquered territory, hastened his operations; to give occupation to the host of adventurers that had assembled under his colors, he dispatched an expedition against the Spanish possessions on Plate River, while in 1643, Johan Maurits equipped the expedition of Hendrik Brouwer that attempted to establish an outpost in southern Chile.

By this series of successful expeditions, he gradually extended the Dutch possessions from Sergipe in the south to São Luís de Maranhão in the north.

[7] His leadership in Brazil inspired two Latin epics from 1647: Caspar Barlaeus' Rerum per octennium in Brasilia et alibi nuper gestarum sub praefectura[8] and Franciscus Plante's Mauritias.

His large schemes and lavish expenditures alarmed the parsimonious directors of the West India Company, and John Maurice, refusing to retain his post unless he were given a free hand, returned to Europe in July 1644.

[10] Shortly after returning to Europe, John Maurice was appointed by Frederick Henry to the command of the cavalry in the Dutch army, and he took part in the campaigns of 1645 and 1646.

When the war was ended by the Peace of Münster in January 1648, he accepted from Frederick William, Elector of Brandenburg (who had just married John Maurice's niece Louise) the post of governor of Cleves, Mark and Ravensberg, and later also of Minden.

Though hampered in his command by the restrictions of the states-general, he repelled the invasion, and the bishop, Christoph Bernhard von Galen, nicknamed "Bommen Berend", was forced to conclude peace.

His campaigning was not yet at an end, for in 1668, he was appointed first field-marshal of the States Army, and in 1673, he was charged by stadtholder William III to command the forces in Friesland and Groningen, and to defend the eastern frontier of the provinces, again against Van Galen.

In the National Library of Paris are two folio volumes containing a fine collection of colored prints of Brazilian animals and plants, which were executed by order of the prince, and accompanied with a short explanation by him.