[5] Bishop Niccolò Albergati was so awestruck with his capabilities that he took him into his service and gave him the chance to pursue his studies further by sending him on a tour through Germany, France and England.

He was to assist in negotiating an understanding between the Papal States and the Holy Roman Empire, regarding undercutting or at least containing the reforming decrees of the Council of Basel (1431–1439).

He called the congress which produced the Treaty of Lodi, secured peace with Charles VII of France, and concluded the Concordat of Vienna or Aschaffenburg (17 February 1448) with the German King, Frederick III,[4] by which the decrees of the Council of Basel against papal annates and reservations were abrogated so far as Germany was concerned.

In the following year he secured a still greater tactical triumph with the resignation of the Antipope Felix V on 7 April and his own recognition by the rump of the Council of Basel that assembled at Lausanne.

The Aqua Virgo aqueduct, originally constructed by Agrippa, was restored by Nicholas and emptied into a simple basin that Alberti designed, the predecessor of the Trevi Fountain.

For Nicholas, humanism became a tool for the cultural aggrandizement of the Christian capital, and he sent emissaries to the East to attract Greek scholars after the fall of Constantinople.

Nicholas, with assistance from Enoch of Ascoli and Giovanni Tortelli, founded a library of five thousand volumes, including manuscripts rescued from the Turks after the fall of Constantinople.

The Roman populace, however, appreciated neither his motives nor their results, and in 1452 a formidable conspiracy for the overthrow of the papal government under the leadership of Stefano Porcari was discovered and crushed.

[6] In late spring of 1452 Byzantine Emperor Constantine XI wrote to Pope Nicholas for help against the impending siege by Ottoman Sultan Mehmed II.

Nicholas issued the bull Dum Diversas (18 June 1452) authorizing King Afonso V of Portugal to "attack, conquer, and subjugate Saracens, pagans and other enemies of Christ wherever they may be found".

[21] Ownership of the Canary Islands continued to be a source of dispute between Spain and Portugal and Nicholas was asked to settle the matter, ultimately in favor of the Portuguese.

[22] The geographical area of the concession given in the bull is not explicit, but historian Richard Raiswell finds that it clearly refers to the recently discovered lands along the coast of West Africa.

[25] The papal bull Romanus Pontifex, issued on 8 January 1455, endorsed Portuguese possession of Cuerta (which they already held), and the exclusive right to trade, navigation, and fishing in the discovered lands, and reaffirmed the previous Dum Diversas.

[26] It granted permission to Afonso and his heirs to "... make purchases and sales of any things and goods, and victuals whatsoever, as it may seem fit, with any Saracens and infidels in said regions; ... provided they be not iron instruments, wood used for construction, cordage, ships, and any kinds of armor.

However, together with a second reference to some who have already been enslaved, this has been used to suggest that Nicholas sanctioned the purchase of black slaves from "the infidel":[30] "... many Guineamen and other negroes, taken by force, and some by barter of unprohibited articles, or by other lawful contract of purchase, have been ... converted to the Catholic faith, and it is hoped, by the help of divine mercy, that if such progress be continued with them, either those peoples will be converted to the faith or at least the souls of many of them will be gained for Christ.

"[27] It is on this basis that it has been argued that collectively the two bulls issued by Nicholas gave the Portuguese the rights to acquire slaves along the African coast by force or trade.

The concessions given in them were confirmed by bulls issued by Pope Callixtus III (Inter Caetera quae in 1456), Sixtus IV (Aeterni regis in 1481), and they became the models for subsequent bulls issued by Pope Alexander VI: Eximiae devotionis (3 May 1493), Inter Caetera (4 May 1493) and Dudum Siquidem (23 September 1493), in which he conferred similar rights to Spain relating to the newly discovered lands in the Americas.