'Intermarium' or 'Meizdymore' in Polish is the idea of a federation consisting of Poland and surrounding countries. The last time Poland was part of something similar was the Polish-Lithuanian Commonwealth.
When that fell, in between the November and January Uprisings, a precursor to Intermarium was an idea by Prince Adam Czartoryski to restore the Polish-Lithuanian Commonwealth, federated with the Czechs, Slovaks, Hungarians, Romanians and all the South Slavs of the future Yugoslavia. Unfortunately due to the many reasons such as lack of Western support and rising German nationalism, the idea was killed off.
During World War I and immediately after it, Josef Pilsudski tinkered with the idea as a counterweight to German and Russian imperialism. His idea: a Intermarium consisting of Lithuania, Estonia, Latvia, Belarus, Poland, Ukraine, Romania and Czechoslovakia. He revised it to be qausi-democratic in nature.
However, no one supported it. The Western Allies believed that Bolshevism was a temporary threat and advised Poland to remain in areas of clear-cut Polish ethnicity, the Lithuanians, Latvians, Ukrainians and Belarussans all feared Polish domination in the supposedly multi-cultural union, not helped by Pilsudski's militaristic expansion of Polish borders into Ukrainian land.
Eventually, this plan was scrapped too.
After the Polish-Soviet War, when it was realized that creating a Polish-Ukrainian axis was impossible, Pilsudski tried again, this time he expanded his plans to include Scandinavia, Yugoslavia and Greece. As you can imagine, this plan flopped too.
To avoid pushing the boundaries of plausibility more than I am already, let's go with Pilsudski's first plan.
What if Pilsudski's first Intermarium came into being in 1919, soon after independence of Poland?