Despite Serbia's rise in power, its ruler remained incompetent. Stefan Uros V's council of advisers continued to rule in his stead, letting him spend most of his time with the army, as he seemed to like meeting with the soldiers and the council saw that as a way of keeping him out of their way. This allowed them to focus more attention on the matter of getting the Serbian state on the right path.
In January of 1367, it was decided that the time was right to press more of Serbia's claims. The principality's finances were in order, the alliance with Poland was strong, and the military of Serbia's target, Tarnovo, was but a pale shadow of that of Serbia itself.
The council's justification for war was based on the brief rule of Stefan Uros V's great aunt, Anna Neda, over Tarnovo. With this basis, they pushed a claim for the province of Badin, though considering the fact that her rule lasted but a year, it was a rather weak one.
The weakness of this claim caused Serbia's ally, Moldavia, to reject the call to declare war on Tarnovo, instead siding with Tarnovo itself. This did not effect Serbia's strategic position in the war, as Moldavia was in no position to help either Serbia or Tarnovo. It did, however, give Poland an opportunity to prove their honor, as they had no means to reach Tarnovo, but did have a convenient border with Moldavia.
The war itself was brutish and short. The main force of the Serbian army, led by Stefan Uros himself, met the Tarnovan forces in Serdicia, where Stefan Uros was able to take advantage of both the terrain and superior numbers to force the Tarnovans into a route. From there it was simply a matter of chasing the remaining forces towards the capital, picking off groups of stragglers until no fighting force remained in the country. All that remained was to capture the border forts and claim both the territory that was rightfully Serbia's.
Noticing the effective example of some of the richer states of Western Europe, the council of advisers, led by Bogdan Hracovec (the natural scientist) Serbia's serfs were given a level of freedom they that had not previously enjoyed. This is widely seen as the basis for the first steps towards the effective, efficient economy that Serbia has today, but at the time it was highly controversial, and caused a great deal of instability both due to the disgruntlement of nobles were used to having more control over those who worked the land and the serfs who were not quite sure of what to do with the freedom they had. In June of 1367, in the midst of the war with Tarnovo, certain peasant leaders came to a conclusion of what to do with their additional freedom: ask for more of it. This ended up being greatly debated by the council, as some feared that the additional rights demanded would weaken Serbia so much that they would not be able to stand up to their enemies. Peasants would refuse to fight in levies, would not stay on their ancestral lands to produce crops for the troops, and would feel more inclined to demand more and more until nothing was left. Bogdan Hracovec was able to effectively and diplomatically assuage these fears, and guided the council to ultimately accept the request, despite any loss in prestige it brought at home and abroad.
The capital of Tarnovo fell in May of 1368, allowing the council to make its demands: In addition to the province of Badin, which Serbia had a (weak) claim to, they also insisted they be given sovereignty over Serdica, claiming it as a historically significant monument of where Serbia won what was by then being referred to as Anna Neda's War. Tarnovo had no choice but to accept this, but Serbia's neighbors largely did not, seeing it as an unpleasant reminder of Serbia's rising ambition.
Not all of Serbia's neighbors found this ambition unpleasant. The very next month, Wallachia accepted an offer of military alliance, providing Serbia with an additional ally in the event of Hungarian hostilities. This alliance proved to be economic as well as military, as the trading of production techniques resulted in brand new breakthroughs in Serbia's olive oil and mining industries.
Serbia had generally made itself a haven of Orthodox peoples living abroad. The late 1300s were a time of great persecution against the Orthodox, as the crusading orders and the Muslims of the Black Sea used force to make adherents of the Orthodox Church convert. In addition to the atrocities committed by these groups, general persecution occurred throughout Catholic Eastern Europe. Serbia generally welcomed the Orthodox refugees into its borders, but twice in the late 1360s were Orthodox refugees turned away. The first time, refugees fled from doctrinal disputes in Wallachia. The council, fearing the effects that letting the refugees into Serbia would have on their relationship, refused them entry. A few years later, Croatian refugees from Hungarian Bosnia similarly sought refuge in Srbija but the council, fearful of Hungary's reaction, refused to allow them to settle in the country.
(Don't ask me why Wallachia was persecuting the Orthodox, I really do not know)
In 1372, the Ottomans long war to conquer southern Greece was complete, with only Constantinople still standing against the Turks. Fearful of the vast and mighty Turkish armies, the council sent multiple wagon trains of tribute to the Turks along with the best Serbian diplomats until they agreed to a non-aggression pact with Serbia.
(I don't find this ominous at all)
The rest of the 1370s were largely a peaceful period for Serbia, focused on economic development and diplomatic stabilization. This peace was broken briefly twice: once when Wallachia pressed its claim on territory belonging to Dobrogea, and the other when Brandenburg attempted to conquer Hinterpommern. In each case, Serbia's participation was minimal. In the first instance, Serbia agreed to defeat Tarnovo in the event it attempted to interfere in the war, but refused to participate in any sieges. In the second instance, Serbia's participation was merely diplomatic; it had no means to get its armies into a position to threaten Brandenburg. Both wars ended up being successful for Serbia's allies. Wallachia seized territory from both Tarnovo and Dogbrogea, and forced Tarnovo into vassalage and Poland was able to force Brandenburg to give up its easternmost province.
(I suspected it might end poorly for Brandenburg)
In 1378, the council felt comfortable enough with the realm's stability to attempt to consolidate power away from the hands of the nobles into that of the office of the Prince. While the motive behind giving more power to someone as weak as Stefan Uros V is questionable, it is more understandable when seen from the perspective that the council of adviser's effectively was using the position of the Prince as an expression of their own power. Unfortunately for them, the nobility of Serbia saw through this power grab. While it was not enough to cause them to revolt, it did cause instability through the realm.
In 1379 the politics of the Mediterranean were changed permanently as Naples won a war with Aragon and was able to claim enough of Sicily to claim the Kingship of the Two Sicilies. This sent shock waves through the region, as the Two Sicilies immediately declared war on the Pope, seeking to bring the southern portion of the Italian Penninsula under their rule.
(This is... unusual.. Also it feels weird to have an Absolute Kingdom in the 1300s)
On October 7, 1379 Stefan Uros V was bit by a snake while touring a new fort in Makedonia. He died within hours at the age of 42.