William began his military career in 1610 in his father's army in the
Upper Palatinate. In 1625 he became
governor of
Emmerich, (which meant they were willing to spend money on an annuity for life). During the
Siege of 's-Hertogenbosch in 1629, William had his
headquarters in , from where
's-Hertogenbosch was first shelled on 15 May. Accompanied by 24
companies of infantry and some
cavalry, William was sent to the
Bommelerwaard by Prince
Frederick Henry of Orange to prevent an invasion by
Hendrik Graaf van den Bergh, then commander of the
Army of Flanders. In the year 1629, William's salary was
£400, equivalent to in , per month. In 1631 William purchased the
heerlijkheid of
Poederoijen on the River
Meuse. William captured the
Kruisschans on the
Scheldt and and other
sconces near
Antwerp, later
Orsoy, in the following year the
Sterreschans and those of
Philippine in . In 1636, the strongly defended
Schenkenschanz was captured from the Spanish by William and his brother
John Maurice. In 1637, William became governor of
Sluis which meant Frederick Henry's entire enterprise failed. To the surprise of his relatives, John 'the Younger' joined the Spaniards in 1617, the opponents of the House of Nassau and the Dutch Republic. In the same year, his older brother John Ernest died in the service of the Republic of Venice. John 'the Middle' had to decide whether an enemy of Nassau and the Netherlands could remain his heir. On 15 November 1617, John declared his will of 8 April 1607 to be null and void. Abolition of the
primogeniture would have meant a division of the small country and John opposed all proposals in that direction. In an amicable agreement, he had his son sign a declaration on 31 December 1617, in which the latter declared that, although he remained a Catholic, he would not force his subjects to any other than the existing religious confession. For John 'the Younger', only one third of the county was provided for in the third will. On 6 August 1621, he was informed of this, with a statement of the reasons that had led his father to take this step. On 9 May 1623, i.e. not until two years later, John 'the Younger' protested against this with a letter from
Frankfurt to the councillors of Siegen. He had not been idle and had not hesitated to denounce his father to the Emperor. At the time of his letter of protest he was certainly already aware of the , which Emperor
Ferdinand II officially issued some time later, on 27 June 1623, informing John 'the Middle' that at the time of making his third will as a fellow combatant of the
outlawed Winter King he was not entitled to make a will. He had to revoke it and answer to an imperial court within two months. It seems that John 'the Younger' then shrank from having the imperial decree delivered to his seriously ill father.
Succession dispute Everyone knew that there would be a dispute at the reading of the will on 11 December 1623. John 'the Younger' had the imperial decree read out and when his brothers demurred, he said as he stood up '' ('The Emperor will part us!'). He had taken the precaution of obtaining a further imperial decree on 20 November 1623 against Countess Dowager
Margaret and her sons, in which the Emperor strictly forbade impeding John's assumption of government, his taking possession of the land and his inauguration. On 12 January 1624, John 'the Younger' was able to accept the homage from the town of Siegen but only because he had secretly let a
squadron of selected horsemen into the town through the castle gate (that is, not through a
city gate) in a heavy snowstorm, so that they could not be seen or heard by the town guards. With the exception of John Maurice and
George Frederick, the younger brothers accepted only modest
appanages. Henceforth, until 1645, the county of Nassau-Siegen had two governments, one in Siegen, the other in Hilchenbach. For a short period (1632–1635) this situation underwent a temporary change, during the
Thirty Years' War, his brothers, who were fighting on the Protestant side, rebelled against John 'the Younger'. Countess Dowager Margaret, through the mediation of Louis Henry, turned to Gustavus Adolphus and asked for help against the machinations of her stepson John 'the Younger'. On 14 February 1632 the
Swedish king sent an order from Frankfurt to Louis Henry to provide military support for his first cousin John Maurice. Louis Henry then occupied the city of Siegen with his
regiment of Dutch and Swedish soldiers. One day later, on 29 February, John Maurice and his brother
Henry arrived in Siegen. Just as John 'the Younger' had kept his cavalry in reserve eight years earlier, now John Maurice and Henry, supported by the presence of the Swedish regiment, negotiated with the
citizens, who felt bound by the
oath they had sworn to John 'the Younger'. On 4 March, after long and difficult negotiations, the citizens paid homage to John Maurice and Henry. As field marshal of the Dutch States Army, he was succeeded by his brother-in-law . ==Marriage and issue==