The encounter of the German Emperor Friedrich I Barbarossa and the Serbian Grand Prince Stefan Nemanja took place in the city of Niš by the end of 12th century. The agreement on military alliance wasn’t reached.
At the beginning of the Third Crusade in 1187, the German Emperor Friedrich I Barbarossa was preparing for the Crusades on Jerusalem that had fallen under the governance of Saladin. On that occasion he was offered help by the Serbian Grand Prince Stefan Nemanja.
The offer came after the Byzantine Emperor made deal with the Hungarian king to end the war and made the arrangements for the matrimony with his daughter Margarita.
The land in the Morava valley that was under Nemanja’s governance would be returned to Byzantium as a dowry. Due to this event Raška lost a powerful ally and the Serbian Grand Prince had to find a new ally.
At that time Friedrich I Barbarossa, who was heading for Jerusalem, decided to take the land route across Buda (Budim), Belgrade, Niš and Constantinople and Stefan Nemanja offered him a safe passage through Raška.
As a part of preparations for the encounter of two rulers that took place in 1189 in Niš, the German delegation was sent to Serbia a year before to negotiate the conditions for the safe passage of the imperial army through Serbia.
The answer was taken to Nürnberg the same year by the Serbian messengers who passed the invitation and the assurances that the German Emperor Friedrich I Barbarossa would be welcomed with all ceremonies in Niš.
“I am preparing the welcoming ceremony in the most famous of all my cities, the one I intend to make the capital of my state”, wrote the Grand Prince to the German Emperor.
By the end of June, 1189 Friedrich I Barbarossa visited first Belgrade and then headed for Niš passing through Braničevo. Here he was welcomed by Stefan Nemanja with his brother, duke Stracimir and nobility.
The negotiations started on July 27th, and since Serbia was at war with Byzantium at the time, Nemanja informed his guest that he intended to continue fighting and that he needed an ally.
Since the only way to form the permanent alliance was to establish a vassal relationship, Nemanja offered such deal to the German Emperor.
The Emperor didn’t want unfriendly relations with Byzantium, so he accepted only some of Stefan Nemanja’s offers.
A “friendship” was established and the alliance was made through the marriage of Nemanja’s nephew Toljen, the son of duke Miroslav of Hum, and the daughter of count Berthold von Andechs, Friedrich I Barbarossa’s cousin.
Leaving Niš, the crusades army continued their trip on Via Militaris to Serdica and Hadrianopolis, followed by Stefan Nemanja and his troupes who conquered the Byzantine lands.
Since there was no danger of being attacked by the German Emperor Friedrich I Barbarossa, the Byzantine Emperor raised his army and headed toward today’s Bulgaria and then Raška and Nemanja who, in the meanwhile, had conquered numerous cities and lands.
The Serbian Grand Prince retreated before the attacks of the Byzantine army only to face them in the battle on the South Morava in 1190 when the Byzantine army took a decisive victory.
The peace according to which Stefan Nemanja had to return the conquered land was signed, but they gave him Kosovo with Lipljan and Metohija all the way to Prizren. It was agreed that he was to be succeeded by his middle son Stefan who was married to Jevdokia, the niece of the Byzantine Emperor.
Text copied from www.serbia.com