Born in Vienna on the 26th of May, 1891. His mother was of Croatian origin and his father was a Polish-born officer of the Austro-Hungarian Army whose only link with Polishness was his genealogy. It was, however, under the influence of his father that Oskar took up studies at the Jagiellonian University. In 1913 he received the doctoral degree on the basis of a thesis on the so-called Sandomierz Agreement (or Sandomierz Consensus; Lat. Consensus Sendomiriensis) in the reign of King Sigismund II Augustus, and was promoted assistant professor three years later. During the First World War he consistently supported the cause of the Polish Legion and the Supreme National Committee. In the independent Second Polish Republic he taught at the University of Warsaw, and at the same time was involved in diplomacy. He was a member of the Polish deputation to the Paris Peace Conference and in the following years he participated in numerous undertakings and institutions connected with the League of Nations. In the interwar period he also firmly established his position as an expert on Central European history, both in Poland and abroad. For instance, he received the honorary doctorates of the universities of Lyon and Montreal. He was also a member of Polish Academy of Arts and Sciences, a co-founder of the Federation of the Historical Societies of Eastern Europe, and participant of international conferences of historians. At that time he got seriously involved in the activities of Catholic intellectuals, becoming, for instance, a member of the Catholic Association for International Research in Freiburg, the chairman of the Union of Catholic Writers, and activist of the ‘Revival’ Catholic Academic Youth Association. The outbreak of the Second World War found him in Switzerland. After the German invasion of France, he left the Old Continent and went to the United States. Together with – among others - Florian Znaniecki and Bronisław Malinowski, he co-founded the émigré Polish Institute of Arts and Sciences of America headquartered in New York and was his chairman in 1953-1962, and honorary chairman from 1964. This institution was supposed to continue the tradition of the Polish Academy o Arts and Sciences. In exile, he vigorously protested against Poland’s gradual subordination to, and falling into dependence on, the Soviet Union, as well as against the changes of Polish eastern border and the political hypocrisy of the Western Allies. Besides, Halecki continued his scientific work in the United States. He was a professor of the Fordham University of New York, but also lectured on the history of East-Central Europe in Montreal, at the University of California, and the Columbia University. He actively supported the Polish American Congress and the Józef Piłsudski Institute of America, too. Much of Halecki’s energy was also devoted to the defence of Catholicism. He reminded Americans of the fate of high-ranking clergymen imprisoned by Communists – for instance, of Cardinal Wyszyński and Cardinal Mindszenty. He also spoke out publicly in defence of Pope Pius XII, and, first and foremost, he organized a campaign aimed at emphasizing the ties binding the Polish State to Christianity and Catholic Church in connection with the millennial celebrations of the Christianization of Poland in 1966. In his assessment of Communism, Halecki represented the school recognizing the Tsarist origin of the Soviets’ Communist imperialism. In his polemics with Józef Mackiewicz, he wrote: “It is true that the former Russian imperialism had never thought of using the international Communism; that, conversely, the Communism is making use of the old imperialism, if necessary not even sniffing at the instrument of Pan-Slavism in its most pro-Russian interpretation. Yet today’s undeniable alliance between the Russian nationalism and Bolshevism has posed not a single, but, unfortunately, a double threat to us, the Poles, and the whole western world” (Wschód europejski, Polska a Rosja, 1957). In that context, Central Europe was not only a certain geopolitical area but also a borderland of civilization. According to Halecki, Russia was a country which – in respect of civilization – was situated outside the limits of Europe. He contrasted the eastern despotism with patterns of voluntary cooperation between states and nations within the framework of wider political systems, the case in point being the Polish-Lithuanian union in the period preceding the partitions of Poland. Thus Halecki was, consistently, a fervent adherent of expanding the political subjectivity of the region stretching between the two imperial powers: Germany and the Soviet Union. Before the Second World War, Halecki devoted much attention in his scientific work to the mediaeval times. One of his most distinguished works dates back to just that period, namely Dzieje Unii Jagiellońskiej (1919-1920), as well as – among others - Wcielenie i wznowienie państwa litewskiego 1386–1401 (1917-1918), Witold (1931), or Polityka skandynawska Jagiellonów (1927). In 1933, the first edition of his great synthesis – Historia Polski (A History of Poland) was published. In the following period Halecki focused upon two questions. On the one hand, he paid much attention to the history of the Church – for instance, in his works such as Unia brzeska w świetle współczesnych świadectw greckich (1954); The Millennium of Europe (1963), or Pierwsze tysiąclecie katolickiej Polski (1966) - particularly in the context of civilizational identity of Poland and Europe. On the other hand, he was engaged in research in the history, and contemporary problems, of the central part of the Old Continent (The Borderlands of Western Civilization. A History of East Central Europe, 1952; Imperialism in Slavic and East European History, 1952; The Limits and Divisions of European History, 1962). His impressive scientific output comprises numerous monographs and articles published in Poland (before the year 1939) and abroad. His Historia Polski has been translated into many languages, including English, French, German, or Italian, and has been reissued as many as about twenty times. Today, Oskar Halecki is the patron of a prestigious Polish competition – History Book of the Year.