The County of Diez (sometimes Dietz, ) was a county of the Holy Roman Empire, centred on the at Diez in Lahngau (in the modern German states of Hesse and Rhineland-Palatinate). The county is first attested in 1073 and seems to have been created from the territory of the Conradine dynasty after they relocated to Swabia. The Counts rose to prominence in the second half of the twelfth century as close partisans of the Hohenstaufen dynasty. During this period, it was known as the "Golden County".
After the fall of the Hohenstaufens in 1254, the power of the Counts of Diez declined. The was created from Diez in 1302 for a collateral line based at . The remaining County of Diez slowly pawned off territories to pay debts until 1388, when the County was inherited by Adolf I, Count of Nassau-Siegen after the death of the last count.
History
The County was probably created as a successor to the original county of the Conradines in Niederlahngau, after they had shifted their centre of power from Franconia to Swabia. The Counts of Diez probably originated in the Nahe river valley and received the county on the Lahn river from the Salian dynasty as a fief, which then became hereditary. The seat was at Diez, from which its territory extended northeast up the Lahn river valley to Weilburg, north into the Westerwald and southeast into the .
The first documentary attestation of the Counts of Diez dates to 1073, when Count Embricho and his brother of Diez (Didesse) sold property in Badenheim to the Stift of in . A Count Embrichio is attested in Niederlahngau from 1059. His brother was probably Count Godebald, who is attested in Niederlahngau from 1053. Embricho, Bishop of Augsburg (1063-1077) was probably a close relative of Count Embricho.
Peak under the Hohenstaufens
Under the Hohenstaufens, especially Frederick Barbarossa, the counts reached the peak of their power. Probably as a result of a marriage to an heiress of the Nürings, Henry II of Diez (r. 1145-1189) inherited substantial possessions in the Wetterau. He accompanied Barbarossa on his Italian Campaigns and participated in diplomatic interactions there, as did his son Henry III. In 1207, Henry III and his brother Gerhard II ceded the Vogtei of Mainz-Kastel to King Philip of Swabia in exchange for imperial immediacy and patronage of the churches at Usingen. Gerhard II also belonged to the regency council of Henry (VII). Henry and Gerhard established the Stift at Salz as a dynastic foundation of the counts of Diez. It was incorporated into the collegiate church of Diez in 1289 by Count Gerhard VII.
The territory was known to contemporaries as the "Golden County". They held the right of the blood court at Altendiez, Flacht, Hahnstätten, Lindenholzhausen, Dauborn, Niederhadamar (Dehrner Zent), Hundsangen, Nentershausen, Meudt, Salz, Rotzenhahn, Hoen-Rennerod, Villmar, Schupbach, Panrod, Kirberg und Camberg, , Elsoff, Blessenberg (Frickhofen) und Niederzeuzheim.
Decline
The decline of the Counts of Diez began at the end of the thirteenth century with the division of its territory with the collateral Weilnau line, based at Altweilnau and then at from 1302. After the end of the Hohenstaufen dynasty, the family lost their influence in Imperial politics. Economic problems followed, which forced the counts to sell off territories.
In 1302, the two lines finally split and formed separate counties. In 1326, the Weilnau line transferred its seat to Birstein in the Vogelsberg. Their territory in the Lahn river valley was mostly taken over by the House of Nassau.
In the following years, the Diez line, weakened by the partition, lost several possessions and privileges, most of which were mortgaged to their more powerful neighbours, Nassau and the Electorate of Trier, as well as the County of Katzenelnbogen and the Lords of Eppstein. Count Gottfried (1303-1348) was ruled to be mentally incapable, so Emicho I, Count of Nassau-Hadamar assumed the guardianship of his fiefs from 1317 to 1332. After 1332, Gottfried's son Gerhard VI acted as regent until his death on 17 October 1343 during a conflict with Limburg.