Wikipedia
Międzymorze , known in English as Intermarium, was a plan, pursued after World War I by Polish leader Józef Piłsudski, for a federation, of Central and Eastern European countries. Invited to join the proposed federation were the Baltic states ( Lithuania, Latvia, Estonia), Finland, Belarus, Ukraine, Hungary, Romania, Yugoslavia and Czechoslovakia.
The Polish name Międzymorze, which means "Intersea" or "Between-seas," was rendered into Latin as "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.
Intermarium complemented Piłsudski's other geopolitical vision— Prometheism, whose goal was the dismemberment of the Russian Empire and that Empire's divestment of its territorial conquests.
Intermarium was, however, perceived by some Lithuanians as a threat to their newly established independence, and by some Ukrainians as a threat to their aspirations for independence, and was opposed by Russia and by most Western powers, except France, who backed it.
Within two decades of the failure of Piłsudski's grand scheme, all the countries that he had viewed as candidates for membership in the Intermarium federation had fallen to the Soviet Union or Nazi Germany, except for Finland (which nonetheless suffered some territorial losses in the Winter War).