A commonwealth that would stretch within the Central- and Eastern Europe is nothing new and have existed since the 1300 century in several different shapes, one example of this is the Polish–Lithuanian union and before the Intermarium was created as an political idea the Polish–Lithuanian–Ruthenian Commonwealth was raised as an possible union, this idea never made it passed the proposal stage but the idea resurfaced several times up until World War I.
The idea of an federation between Central- and Eastern European countries was born after the World War I and led by Józef Piłsudski a polish politician, this federation would be better known as the Intermarium and was supposed to stretch between the Baltic- and Black Sea. The federation would work together on an economical- and military level to secure it member nation from the Russian Empire and ensure the newly won freedom for their member states. This idea never gained any momentum due to distrust from several of the countries that recently gained their freedom and therefor became dormant until the 1930’s when yet another try was made, but it failed this time as well.
It wasn’t until the after the fall of the Warsaw pact that the idea for an Intermarium would start to gain momentum but it would take another 20 years before the idea grew into real cooperations between countries, one of these cooperation would be the Visegrád Group consisting of Czech Republic, Hungary, Poland and Slovakia.