[5] John Ernest served in the Dutch States Army under Maurice of Nassau since 1601, where he took part in the Siege of Ostend.

In a letter of 8 December 1608 he even considered the death of Adolf as a punishment from God and he exhorted the two others, who with "einem ärgerlichen Leben mit Verschwendung fast allem, was ich in der Welt habe, durch Ehebrechen und Hurerei, Plünderung und Beraubung armer, unschuldiger Leute hoch und niederen Standen" ("an annoying life of squandering almost everything I have in the world, through adultery and fornication, plundering and robbing poor, innocent people of high and low rank") ruined the county of Nassau-Siegen, to lead a different, better life, worthy of the name Nassau.

To avoid this, on 8 April 1607 he made a will and testament, which stated that only the eldest son would rule, and the other children should be compensated with money or offices.

[20] The signing of the Twelve Years' Truce and the end of the War of the Jülich Succession deprived John Ernest of the opportunity to distinguish himself further.

[21] John Ernest obtained permission from the States General in a resolution of 3 October 1616 to enter the service of the Venetian Republic with a regiment of foot soldiers.

[21] On 2 March 1617, John Ernest, at the head of his soldiers, left the shipstead of Texel and, after a prosperous journey, arrived in Venice on 4 April.

[11] John Ernest arrived with his troops at Gradisca, a small but strong Austrian town in the County of Gorizia, on the River Isonzo, which had been besieged in vain by the Venetians for a long time.

Initially, two enemy sconces were conquered by the Dutch, but when John Ernest wanted to take advantage of the terror of the besieged, the commander of the Venetians prevented him from doing so.

John Ernest remained with his troops before Gradisca, and no doubt this city would have collapsed if the peace had not been signed shortly before.

[23] John Ernest died in Udine in the night of 16/17 September 1617Jul..[1][note 5] His body was embalmed, and taken in a coach to Venice.

[25] From there, his body was taken at the expense of the Venetian Republic to Siegen, where it was interred on 19 April 1618 under the choir of the St. Nicholas Church [de].