Wallachia's borders moved constantly throughout history, but during his ruling, Wallachia had the widest surface in the medieval times: from river Olt in the north to Danube in the south, and from Danube's Iron Gates in the west to the Black Sea to the east.
Mircea strengthened the power of the state and organized the different high offices, promoted economic development, increased the state's revenue, and minted silver money that enjoyed wide circulation not only inside the country but also in the neighbouring countries. He gave the merchants of Poland and Lithuania trade privileges and renewed those his predecessors had given to the people of Braşov. Mircea could thus afford to increase his military power. He fortified the Danube citadels and strengthened "the great army" made up of townspeople and of free and dependent peasants. He also proved a great support for the Church.
While organizing the country, he also took good care to form a system of lasting alliances that might enable him to defend the independence of the country. Through the intermediary of Petru Muşat, ruling prince of Moldavia, he concluded in 1389 a treaty of alliance with Wladyslaw II Jagiello, king of Poland. The treaty was renewed in 1404 and 1410. He maintained close relations with Sigismund of Luxembourg, the king of Hungary, relying on their common interest in the struggle against Ottoman expansion.
His interventions in support of the Christian peoples south of the Danube who were fighting against the Turks, brought him into conflict with the Ottoman Empire. In 1394 Beyazid I (a.k.a. "Beyazid Ilderim", "the Thunderbolt") crossed river Danube leading 40,000 men, an impressive force at the time. Mircea had only about 10,000 men so he couldn't survive an open fight. He chose what we would call guerrilla warfare today, by starving the opposing army and by small, localized attacks and retreats (a typical form of asymmetric warfare). On October 10 1394, the two armies finally clashed at Rovine, which features a forrested and swampy terrain, thus preventing the Ottomans from properly spreading their army; Mircea finally won the fierce battle and threw the Ottomans out of the country (the battle was epically described by Mihai Eminescu in his Third Epistle).
Helped by Sigismund of Luxemburg, Mircea then also got rid of Vlad Uzurpatorul, a puppet of the Ottoman's Sultan, seeking the throne. In 1396 Mircea participated in an anti-Ottoman crusade started by Hungary's monarch. The crusade ended with the Ottoman victory on September 25. Next year, 1397, Mircea stopped another Ottoman expedition which crossed the Danube, and in 1400 he defeated yet another expedition of Turks crossing the country.
The defeat of Sultan Beyazid I by Timur Lenk (Tamerlane) at Ankara in the summer of 1402 opened a period of anarchy in the Ottoman Empire and Mircea took advantage of it to organize together with the Hungarian king a campaign against the Turks. In 1404 Mircea was thus able to impose his rule on Dobrogea again. He moreover took part in the struggles for the throne of the Ottoman Empire and enabled Musa to ascend that throne (for a brief reign). It was at this time that the prince reached the height of his power.
The "bravest and ablest of the Christian princes", as he was described by German historian Leunclavius, has ruled Wallachia for 32 years. Apart from his military successes Mircea was an art lover, leaving us among other monuments beautiful Cozia Monastery, built after the model of the Krusevac Church in Serbia.
Towards the end of his ruling, Mircea signed a treaty with the Ottomans who recognized the freedom of Wallachia in return for a 3,000 gold pieces per year tribute (a small amount). Keeping the country free from becoming an Ottoman province ('pashalik') also meant keeping the Ottomans away from Western Europe, a feat which retrospectively might not sound impressive, but was a very important accomplishment at the time.