Rene de Chalons, Prince of Orange, made his will because he was going into battle. As he was young and married only a short while, he did not expect his cousin, 11-year-old William of Nassau, to become
his heir; but he nominated him and as he fell in the battle of St.Dizier in Champagne (18 July 1544) William, Count of Nassau became Prince of Orange. This was why William's younger brother, Johann,
became their father's heir; Johann then in due course became Count of Nassau-Dillenburg, Katzenelnbogen, Dietz, Siegen, Hadamar and Beilstein.
Even though he was in Germany, Johann always supported his elder brother's efforts in The Netherlands. At first this support was only financial, but later he also supplied a home for William's family when they had to flee the war-torn Netherlands.
At this time he was involved with the delicate matter of his brother William's divorce. William as a 27-year-old widower had married (24 August 1561) the 16-year-old Anna of Saxony. Already unstable, she felt neglected by her husband's state affairs; however, five children were born of which three survived. Then she forced her attentions upon Jan Rubens, a lawyer looking after her affairs. In her divorce it was stated that, on the 2nd of June 1570 at Ebersbach, they had become lovers and this was repeated "twelve or fourteen" times. When rumours reached William and Johann, they imprisoned Jan Rubens; Anna at first denied everything, but when Jan Rubens wrote a letter admitting their affair, she, too, confessed.On 22 August 1571, at the castle of Siegen, Anna gave birth to a daughter, Christina von Dietz. After the divorce Anna was kept in custody when she became violent and even more deranged, and died at only 33 on 18 December 1577.
After continuous pleading by his wife, Jan Rubens was released on 10 May 1572, when they went to Siegen where, on 29 June 1577, their son, Peter Paul Rubens the painter, was born.
When the situation in The Netherlands improved, Johann, too, moved to The Netherlands to become 'Stadhouder' of Gelderland (1578-1581).
In 1579 he was the instigator of the "Union of Utrecht" which may be regarded as the foundation of The Netherlands as an independant nation free from Spain. This no doubt, is Johann's greatest achievement. 
In 1581 he returned to Germany where, in 1584, he was the founder of a Gymnasium (high school) in Herborn. By 1590 The Netherlands were in a better financial state, and Johann requested to have returned the money he had lent for his brother's war against Spain. On 31 July 1594 an agreement was made but it would take thirty years before it would be finalised.
Johann, the fifth of his father's fifteen children, died 70 years old, having survived all his brothers and sisters. Of the five brothers, William had been murdered (1584), on instigation of Philip II of Spain; Adolf fell in battle in 1568; and both Ludwig and Heinrich fell in battle in 1574. Johann was the only one to die a natural death.
 

Source: Leo van de Pas