William I, Count of Nassau-Dillenburg (1487–1559), called the rich, owned the counties of Nassau-Dillenburg, Nassau-Siegen, Nassau-Diez and Vianden.
His eldest son William the Silent inherited the principality of Orange in Southern France from his cousin René of Chalon, as well as from his father the vast properties of the House of Nassau-Dillenburg in the Netherlands, which Engelbert I of Nassau had received by marriage in 1403.
William the Silent originally served the Habsburgs as governor of the Spanish Netherlands, but later defected and became the main leader of the Dutch revolt against the Spanish Habsburgs that set off the Eighty Years' War and in 1581 resulted in the formal independence of the United Provinces of which he became the stadtholder.
He was the founder of the House of Orange-Nassau.

The early House of Orange-Nassau extinguished in the male line with William III of England (d. 1702), while the later House of Orange-Nassau (and the present Dutch royal family) descend in the male line from William I's younger brother John, Count of Nassau-Dillenburg, and subsequently from the latter's fifth son, Count Ernst Casimir of Nassau-Dietz.
The Nassau-Dietz branch is also known as the "Friesian branch" because its ruling counts served as stadtholders in Friesland and Groningen at the same time.

In 1702, the Nassau-Dietz branch followed the House of Orange that had become extinct.
The counts of Nassau-Dietz not only descended from William I., the Silent's, brother, but in female line also from himself, as William Frederick, Prince of Nassau-Dietz, had married Countess Albertine Agnes of Nassau, the fifth daughter of Frederick Henry, Prince of Orange in 1652.
She had Schloss Oranienstein built from 1672 as her new residence at Diez.

Their grandson Johan Willem Friso (1687-1711) became Stadholder in Friesland and Groningen, and in 1702 became the heir of William III of England and thus the founder of the younger House of Orange-Nassau and of the Dutch Royal Family.
However, he had to split the Dutch properties with the King of Prussia who also descended from William I. Johan Willem Friso's son, William IV, Prince of Orange, inherited a number of Nassau territories besides his paternal Nassau-Dietz, namely Nassau-Hadamar in 1711, Nassau-Siegen in 1734, and Nassau-Dillenburg in 1739.
In 1732, Frederick William I of Prussia left him his Dutch properties, including Huis ten Bosch palace and Het Loo Palace.
William IV became stadtholder of the Netherlands in 1747 and reunited all of the Dutch and German possessions of his family (except for Nassau-Weilburg) in his hand, styling himself Prince of Orange and Nassau.

The county of Nassau-Diez, like other Nassau territories, was occupied by Napoleonic France in 1795 and in 1806 was annexed by the Duchy of Nassau (ruled by the branch of Nassau-Weilburg) on 16–17 September 1796 as a consequence of the 2nd Coalition war between Austrians and French in the area between Diez and Limburg.
By the end of the 18th century the entire west bank of the Rhine went to France and in 1803 a new Principality of Orange was formed from other territories, however only to be divided between the Duchy of Nassau and the Grand Duchy of Berg in 1806.
William I of the Netherlands recovered his former counties in 1813, but gave Nassau-Diez, Nassau-Hadamar und Nassau-Dillenburg to Prussia, in exchange with Luxembourg, two years later.
Prussia kept only Nassau-Siegen and soon ceded the other counties to the Duchy of Nassau which was however annexed by Prussia, including Diez, after the Austro-Prussian War in 1866, for Nassau's support of Austria.

As a result of the Prussian Administrative Reforms of 1867 the Lahn district was created with the county seat of Diez.
It Initially belonged to the lower Lahn Limburg circle.
The episcopal city became, however, in 1886 the seat of the newly established district of Limburg.
In the course of municipal reform in 1969 Loreley circle the lower Lahn (located in St. Goarshausen) Rhein-Lahn-Kreis merged with the spa town of Bad Ems, which became the country seat.
William I, Count of Nassau-Dillenburg (1487–1559), called the rich, owned the counties of Nassau-Dillenburg, Nassau-Siegen, Nassau-Diez and Vianden. His eldest son William the Silent inherited the principality of Orange in Southern France from his cousin René of Chalon, as well as from his father the vast properties of the House of Nassau-Dillenburg in the Netherlands, which Engelbert I of Nassau had received by marriage in 1403. William the Silent originally served the Habsburgs as governor of the Spanish Netherlands, but later defected and became the main leader of the Dutch revolt against the Spanish Habsburgs that set off the Eighty Years' War and in 1581 resulted in the formal independence of the United Provinces of which he became the stadtholder. He was the founder of the House of Orange-Nassau. 
The early House of Orange-Nassau extinguished in the male line with William III of England (d. 1702), while the later House of Orange-Nassau (and the present Dutch royal family) descend in the male line from William I's younger brother John, Count of Nassau-Dillenburg, and subsequently from the latter's fifth son, Count Ernst Casimir of Nassau-Dietz. The Nassau-Dietz branch is also known as the "Friesian branch" because its ruling counts served as stadtholders in Friesland and Groningen at the same time. 
In 1702, the Nassau-Dietz branch followed the House of Orange that had become extinct. The counts of Nassau-Dietz not only descended from William I., the Silent's, brother, but in female line also from himself, as William Frederick, Prince of Nassau-Dietz, had married Countess Albertine Agnes of Nassau, the fifth daughter of Frederick Henry, Prince of Orange in 1652. She had Schloss Oranienstein built from 1672 as her new residence at Diez. 
Their grandson Johan Willem Friso (1687-1711) became Stadholder in Friesland and Groningen, and in 1702 became the heir of William III of England and thus the founder of the younger House of Orange-Nassau and of the Dutch Royal Family. However, he had to split the Dutch properties with the King of Prussia who also descended from William I. Johan Willem Friso's son, William IV, Prince of Orange, inherited a number of Nassau territories besides his paternal Nassau-Dietz, namely Nassau-Hadamar in 1711, Nassau-Siegen in 1734, and Nassau-Dillenburg in 1739. In 1732, Frederick William I of Prussia left him his Dutch properties, including Huis ten Bosch palace and Het Loo Palace. William IV became stadtholder of the Netherlands in 1747 and reunited all of the Dutch and German possessions of his family (except for Nassau-Weilburg) in his hand, styling himself Prince of Orange and Nassau. 
The county of Nassau-Diez, like other Nassau territories, was occupied by Napoleonic France in 1795 and in 1806 was annexed by the Duchy of Nassau (ruled by the branch of Nassau-Weilburg) on 16–17 September 1796 as a consequence of the 2nd Coalition war between Austrians and French in the area between Diez and Limburg. By the end of the 18th century the entire west bank of the Rhine went to France and in 1803 a new Principality of Orange was formed from other territories, however only to be divided between the Duchy of Nassau and the Grand Duchy of Berg in 1806. William I of the Netherlands recovered his former counties in 1813, but gave Nassau-Diez, Nassau-Hadamar und Nassau-Dillenburg to Prussia, in exchange with Luxembourg, two years later. Prussia kept only Nassau-Siegen and soon ceded the other counties to the Duchy of Nassau which was however annexed by Prussia, including Diez, after the Austro-Prussian War in 1866, for Nassau's support of Austria. 
As a result of the Prussian Administrative Reforms of 1867 the Lahn district was created with the county seat of Diez. It Initially belonged to the lower Lahn Limburg circle. The episcopal city became, however, in 1886 the seat of the newly established district of Limburg. In the course of municipal reform in 1969 Loreley circle the lower Lahn (located in St. Goarshausen) Rhein-Lahn-Kreis merged with the spa town of Bad Ems, which became the country seat. 
Diez, Germany