Being the king of Poland, Jogaila could not rule the duchy of Lithuania directly any more and he appointed one of his brothers, Skirgaila, the grand duke of Lithuania. Other Lithuanian dukes opposed this decision and in 1392 the son of Kestutis, Vytautas, replaced Skirgaila as the grand duke and a vassal of Jogaila. He pacified the other dukes and got rid of the personal dependence of Jogaila. On the congresses of the Polish and Lithuanian nobles in 1401 and 1413 the dynastic union was confirmed and Vytautas was recognized as the ruler of his own small duchy only. However, his unusual talents allowed him to become the direct heir of Gediminas and Olgerdas. In 1395 he adjoined the Smolensk principality to Lithuania, the territory of Lithuania grew and spread from the Baltic to the Black sea. Trying to increase his influence, Vytautas interfered the affairs of Novgorod, Pskov, Tver, Moscow, Ryazan and other Russian principalities. River Ugra (a tributary of Oka) became the border between Lithuania and the lands of the grand knyaz of Moscow Vasily, Vytautas' son-in-law. Vytautas even made some attempts to establish control over the Golden Horde, which was torn apart by the feuds, but the ruler of the Horde Edigu defeated Vytautas on river Vorskla and put an end to the expansion of Lithuania. This time was later thought to be the Golden Age of Lithuania. However, this was also the time when the first signs of the decay were noticed.
The strengthening of Vytautas was supported by those Lithuanians who opposed the union with Poland. One might think that Vytautas could leverage this support to make his duchy another Russian state, like Muscovy, and, probably, even unite the whole Rus under his rule. But Vytautas needed the support of Poland against the Teutonic Order. Besides, a new large group of population appeared in Lithuania, the proponents of the union. These people were motivated by the terms of the union defined by the Polish-Lithuanian congress of 1413 that took place in Horodlo: the Lithuanians who baptized in Catholicism, were granted the rights and privileges of the equal Polish estate. So, former members of the duke's druzhina entered the szlachta, the Lithuanian court copied the Polish one, and the top positions were given to Catholics only. This was a good opportunity for those who didn't mind to convert.
So, there were not two, but three parties now among the retinue of Vytautas: Orthodox-Russian, old Lithuanian and pro-Catholic. All of them were sure that the duke is on their side, but he never directly supported any of these parties. He might support the pro-Polish party, but they were still too weak. In the end of his life Vytautas even attempted to obtain the king's crown from the emperor of the Holy Roman Empire, but failed and died in 1430, being unable to unite the three parties. The struggle between these parties became later the reason of the fall of the greatl Lithuanian-Russian duchy.