The heir of Vasily the Dark was his son, Ivan Vasilyevich. Vasily had made Ivan his co-ruler and gave him the title of the grand knyaz before Vasily died. Having grown up in the hard times, clever boy became experienced in politics. When he became the knyaz, his lands were surrounded by other Russian principalities: Novgorod, Tver, Rostov, Yaroslavl, Ryazan. He subjugated them either by force or by peaceful agreements and in the end of his rule Rus only bordered with other peoples: Swedes, Germans, Lithuanians, Mongols. In the early years, Ivan was one of many Russian knyazes. Now that he put them under his rule, he became the lord of the whole nation. His politics inevitably had to change from feudal to national.
This change made him oppose the old system of appanage and vassalage. He limited the rights of lesser rulers. In his will, he left the throne to his elder son Vasily and gave nothing to the younger sons, making them simple aristocrats in the service of the grand knyaz. This was the politics of monocracy, when the grand knyaz was the single ruler of all the people, including other knyazes and boyars. The monocracy led to the changes in his court. The growing importance of the grand knyaz had to be expressed in pompous rituals, multiple titles and ranks of the servants, various symbols of the lord's power.
The foreign politics had also changed. Rus was totally liberated from the Mongolian occupation. The defensive attitude towards the expansion of Lithuania was replaced by active politics. Ivan III claimed the Russian lands that were ruled by Lithuania since Gediminas. He also led active politics against the Livonian Order.
The unification of the Northern Rus was started under Dimitri Donskoy (Dimitri of Don) and finished under Ivan III. So, Ivan III may be named the founder of Muscovy.