Intermarium is a geopolitical idea that was popular in Poland between the World Wars, but it was based on much older political considerations hailing back to the late 14th century and used to good effect in the two following centuries, then brought up again in mid-19th century when the Poland vanished from the map as an independent state and is still very much alive today, although in slightly different form.

The idea of 'intermarium' or 'land between the seas' is, in its simples form, a geopolitical idea of the block of allied powers controlling the area from the Baltic Sea in the north to the Black Sea in the south (some variations also included Balkans adjacent to Adriatic sea in the south-west) to facilitate common interests and defence against other neighbouring powers or similar blocks of countries sharing the policy. The beginnings of the concepts are usually linked to the beginning of the Jagiellon dynasty of Poland, founded by Władyslaw Jagiello (Jogaila), a Lithuanian Grand Duke (effectively a king) who ascended to the throne of Poland marrying Hedwig of Anjou, daughter of Louis I, king of Hungary, essentially consolidating the lands of Poland and Lithuania and maintaining good relations with Kingdom of Hungary. The Polish-Lithuanian union was precipitated by the quick development of the common rival in the area, that is the Teutonic Order. Intermarriages of the royal families ruling Poland, Lithuania, Hungary and Bohemia resulted in the period of common interests and steady development of these Central European countries finally ended in 1526 with the death of Louis II, king of Hungary, Bohemia and Croatia in the battle of Mohacs. This allowed Habsburgs to get control over Bohemia, Croatia and Hungary thanks to the fact that Ferdinand Habsburg was a brother-in-law of the deceased king and managed to secure the support of local nobility. Southern part of Hungary was also seized by Ottoman Empire that would remain an important threat to Poland, Lithuania and Habsburg lands for the next one and a half century. With the last of Jagiellons dying in 1572 with no issue, little hope of getting Bohemia, Hungary and Croatia back into Polish-Lithuanian sphere of influence and the adoption of the electional monarchy, concept of 'intermarium' has been abandoned. The fact that after slightly over two centuries Poland-Lithuanian Commonwealth will get partitioned by the neighbouring countries may serve as the its proof of validity, however.

After the failed November Uprising and resulting Polish-Russian War (1830-1831), Polish emigrant politicians led by the conservative activist duke Adam Czartoryski, leader of the conservative-liberal, monarchistic faction operating in France and striving to gain the support of the foreign powers (mainly England, France and Ottoman Empire) to restitute Poland as a sovereign country. Czartoryski proposed the re-creation of independent Polish-Lithuanian Commowealth that would be confederated or at least allied with other Central European nations, including Czechs, Slovaks, Hungarians, Romanians, Serbians, Slovenians, Croats and others (this was a far-fetched project, although the independent nationalistic tendencies in the area were clearly visible in the Spring of Nations period of the late 1840's). The idea was to create a coherent block of mostly Slavic countries that would be a counterweight to Germanic countries in the west and Russian Empire to the east. This vision was never realised though, mainly due to lack of Western support, but even if Poland was restituted, chances of coalition were slim due to territorial conflicts between Hungary, Bohemia, Slovakia and Romania.

Pilsudski's idea was at the core identical to one of Czartoryski. The 'intermarium' was envisioned by him as a way to hold off the expansive, militant Germany and Russia (later USSR). Piłsudski, having lived in Russia where he belonged to a militant separatistic organisation and later having served in Austrian army was well-versed in social and political problems apparent in both these countries. He believed that the only way to thwart the danger of German and Russian expansionism was to create a block of countries able to cooperate against two powers mentioned above (Piłsudski himself thought that Russia is more dangerous of the two, especially after Polish-Bolshevik War of 1920). His political goal was to weaken the Russia (later Soviet Union) by federating Lithuania, Ukraine and Poland, thus forming the Polish-Lithuanian-Ukrainian Commonwealth, even at a cost of ceding part of Polish territory to Lithuania and Ukraine. This attempt was thwarted by the nationalistic and independentionist tendencies in Lithuania and Ukraine with the local politicians and their supporters fearing Polish hegemony that could endanger their newly gained sovereignty. The idea of federation was also opposed by National Democrats united around Roman Dmowski, who were supporting the idea of monoethnic country, fearing that due to changes in the national identity of Lithuanians and Ruthenians, the possibility of creating well-functioning parliament in multi-ethnic country is slim at best. Yet in 1931, with the publications of the book 'Imperium Jagiellonicum: A discourse of East-European Union' by Stefan Gużkowski, the idea of the federation encompassing lands from Finland to the north to Greece in the south re-emerged. Józef Beck, Polish Minister of Foreign Affairs came up with the concept of the coalition of the Central European states ('The Third Europe') acting as a geopolitical counterbalance to Western Powers (England and France), Fascist states (Third Reich and Italy) and the USSR, with the core of the coalition formed of Poland, Hungary and Romania and further incorporating Czechoslovakia, Yugoslavia and, if possible, Italy, Lithuania, Latvia, Estonia and Finland. Like Piłsudski's federational idea, this plan also didn't come through, mainly due to political goals of prospective allies. Italy remained on Germany's side, cooperation of Romania and Hungary was marred by conflict concerning Transylvania, Italy and Hungary were not hinding the wish to divide Yugoslavia between themselves and virtually no country was willing to accept the leading role of Poland. After the breakout of the war in 1939, the idea was discussed as a potential post-war solution, strongly advocated by general Władysław Sikorski. In November 1942 Poland and Czechoslovakia officially sign the act of federation, but the tragic death of Sikorski in 1943 and the emergence of post-war bipolar geopolitics that divided Europe between Western countries allied with USA on one side and USSR with its satellite states on the other made the concept impossible to realise.

The fall of the Soviet Union and regaining of the independence by the former satellite states brought back the idea of intermarium (or more accurately, of the cooperation between Central European states). Even before the collapse of USSR, cooperation between the countries of Central and Southern Europe was proposed by Austria, Yugoslavia, Italy and Hungary in 1989, forming the Central Europe Initiative, commonly knowns as Quadragonale and later as Pentagonale and Hexagonale with Czechoslovakia joining in 1990 and Poland in 1991. The idea of CEI was quite similar to the 'intermarium' concept, to wit the prevent the power vacuum created by the collapse of the USSR, create the counterweight to potential German economical expansion into Central Europe and help former Soviet satellite states to integrate with Western political structures such as EEC (later EC and finally EU). Echoes of Jagiellonian policy can be also found in the formation of Vysehrad Group by Hungary, Czechoslovakia and Poland in 1991 with the goal of cooperation aimed at maintaining ties with Western Europe, that culminated in 2004 with admission of all the four member states (Czechoslovakia divided into Czech and Slovakia in the meantime) into European Commonwealth (now European Union).