The Polish name Międzymorze may be translated as "Intersea" or "Between-seas" and has also been rendered, from the Latin, as "Intermarum" or "Intermarium."
The proposed federation was meant to emulate the Polish-Lithuanian Commonwealth, stretching from the Baltic Sea to the Black Sea, that, from the end of the 16th century to the end of the 18th, had united the Kingdom of Poland and the Grand Duchy of Lithuania.
Międzymorze was, however, perceived by some Lithuanians as a threat to their newly established independence, seen by some Ukrainians as a threat to their aspirations for independence, and opposed by Russia and by most western powers except France.
Międzymorze complemented Piłsudski's other ambitious geopolitical vision—Prometheism, whose goal was no less than the dismemberment of the Russian Empire and that Empire's divestiture of its territorial conquests . .
Within two decades of the failure of Piłsudski's grand scheme, all the countries that he had viewed as candidates for membership in the Międzymorze federation had fallen to the Soviet Union or Nazi Germany.
A Polish-Lithuanian union and military alliance had come about as a mutual response to a common threat from the Teutonic Order. The alliance was first established in 1385 by the wedding of Poland's Queen Jadwiga and the Gediminid dynasty in the person of Lithuanian Grand Duke Jogaila, who became King Władysław II Jagiełło of Poland. A longer-lasting federation was established via the creation of the Polish-Lithuanian Commonwealth, an arrangement that lasted until the late-18th-century Partitions of Poland.
Between the November and January Uprisings, in 1832–61, the idea of resurrecting an updated Polish-Lithuanian Commonwealth was advocated by Prince Adam Jerzy Czartoryski, residing in exile at the Hôtel Lambert in Paris.
In his youth Czartoryski had fought against Russia in the Polish-Russian War of 1792 and would have done so again in the Kościuszko Uprising of 1794, had he not been arrested at Brussels on his way back to Poland. Subsequently in 1795 he and his younger brother had been commanded to enter the Russian army, and Catherine the Great had been so favorably impressed with them that she had restored to them part of their confiscated estates. Adam Czartoryski had subsequently served Tsars Paul and Alexander I as a diplomat and foreign minister, during the Napoleonic Wars establishing an anti-French coalition. Czartoryski had been one of the leaders of the Polish November 1830 Uprising and, after its suppression by Russia, had been sentenced to death but eventually allowed to go into exile in France.
In Paris the "visionary statesman and former friend, confidant and de facto foreign minister of Russia's Tsar Alexander I acted as the "uncrowned king and unacknowledged foreign minister" of a nonexistent Poland.
In his book, Essai sur la diplomatie (Essay on Diplomacy), completed in 1827 but published only in 1830, Czartoryski observed that, "Having extended her sway south and west, and being by the nature of things unreachable from the east and north, Russia becomes a source of constant threat to Europe." He argued that she would have done better, cultivating "friends rather than slaves." He also identified a future threat from Prussia and urged the incorporation of East Prussia into a resurrected Poland.
His diplomatic efforts anticipated Piłsudski's Prometheist project in linking efforts for Polish independence with similar movements of other subjugated nations in Europe and in the east, as far as the Caucasus.
Czartoryski aspired above all to reconstitute — with French, British and Turkish support — a Polish-Lithuanian Commonwealth federated with the Czechs, Slovaks, Hungarians, Romanians and all the South Slavs of the future Yugoslavia. Poland, in his concept, could have mediated the conflicts between Hungary and the Slavs, and between Hungary and Romania. The plan seemed achievable during the period of national revolutions in 1848-49 but foundered on lack of western support, on Hungarian intransigence toward the Czechs, Slovaks and Romanians, and on the rise of German nationalism.
"Nevertheless," concludes Dziewanowski, "the Prince's endeavor constitutes a [vital] link [between] the 16th-century Jagiellon [federative prototype] and Józef Piłsudski's federative-Prometheist program [that was to follow after World War I].
Józef Piłsudski's strategic goal was to resurrect an updated form of the Polish-Lithuanian Commonwealth, while working for the disintegration of the Russian Empire, and later the Soviet Union, into its ethnic constituents (the latter was his Prometheist project). Piłsudski saw the Międzymorze federation as a counterweight to imperialist tendencies on the part of Russia and Germany. According to Dziewanowski, the plan was never expressed in a systematic fashion, and relied on Piłsudski's pragmatic instincts. The British scholar George Sanford has asserted that Pilsudski recognized the plan's infeasibility at around the time of the Polish-Soviet War of 1920.
Piłsudski's concept was opposed within Poland itself, where National Democracy leader Roman Dmowski argued for an ethnically purer Poland in which minorities would be Polonized. Many Polish politicians, including Dmowski, opposed the idea of a multicultural federation, preferring instead to work for a unitary Polish nation-state. Sanford has described Pilsudski's policy after his resumption of power in 1926 as similarly focusing on the Polonization of Eastern Slavic minorities and centralization of power.
While some scholars accept at face value the democratic principles claimed by Piłsudski for his federative plan, others view such claims with skepticism, pointing out that in later life Piłsudski would become increasingly disillusioned with democracy as he observed its operation in interbellum Poland; he staged a coup d'etat in 1926 and assumed dictatorial powers. In particular, his project is viewed unfavorably by most Ukrainian historians, with Oleksandr Derhachov arguing that the federation would have created a greater Poland in which the interests of non-Poles, especially Ukrainians, would have gotten short shrift.
Some historians hold that Piłsudski, who argued that "There can be no independent Poland without an independent Ukraine," may have been more interested in splitting Ukraine from Russia than in assuring Ukrainians' welfare. He did not hesitate to use military force to expand Poland's borders to Galicia and Volhynia, crushing a Ukrainian attempt at self-determination in disputed territories east of the Western Bug River which contained a substantial Polish minority, mainly in cities like Lwów (Lviv), but a Ukrainian majority in the countryside.
Speaking of Poland's future frontiers, Piłsudski said: "All that we can gain in the west depends on the Entente — on the extent to which it may wish to squeeze Germany," while in the east "there are doors that open and close, and it depends on who forces them open and how far." In the eastern chaos, the Polish forces set out to expand as far as feasible. On the other hand, Poland had no interest in joining the western intervention in the Russian Civil War or in conquering Russia itself.
Piłsudski next contemplated a federation or alliance with the Baltic and Balkan states. This plan envisioned a Central European union including Poland, Czechoslovakia, Hungary, Scandinavia, the Baltic states, Italy, Romania, Bulgaria, Yugoslavia and Greece — thus stretching not only west-east from the Baltic to the Black Sea, but north-south from the Arctic Ocean to the Mediterranean Sea. This project also failed: Poland was distrusted by Czechoslovakia and Lithuania; and while it had relatively good relations with the other countries, they had tensions with their neighbors, making it virtually impossible to create in Central Europe a large block of countries that all had good relations with each other. In the end, in place of a large federation, only a Polish-Romanian alliance was established.
Piłsudski died in 1935. A later version of his concept was attempted by interwar Polish Foreign Minister Józef Beck, a Piłsudski protégé, whose proposal during the late 1930s of a "Third Europe" — an alliance of Poland, Romania and Hungary — also gained little ground before World War II supervened.
The concept of a "Central European Union" — a triangular geopolitical entity anchored in the Baltic, Black, and Adriatic or Aegean Seas — was revived during World War II in Władysław Sikorski's Polish Government in Exile. A first step toward its implementation — 1942 discussions between the Greek, Yugoslav, Polish and Czechoslovak exile governments regarding prospective Greek-Yugoslav and Polish-Czechoslovak federations — ultimately foundered on Soviet opposition, which led to Czech hesitation and Allied indifference or hostility.
Other forms of the concept have survived into the late 20th and early 21st centuries, including regional-security proposals that were not framed as being Polish-led. Poland's neighbors, however, continued to perceive the idea as imperialist.
After the Warsaw Pact collapsed, Poland, Hungary and the Czech Republic joined NATO in 1999, and Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia joined in 2004. Ukraine has expressed interest in joining as well.