The reason I started this topic is to give modders better "feel" of Poland 1918-39; I'll describe here briefly Polish foreign affairs and military doctrine, hope it'll help some modders
Poland before 1918
Poland had long tradition, but ceased to exist in 1795, as a result of partition between Prussia, Russia and Austria. During napoleonic period we allied ourselves to Napoleon and, of course, lost. Later on polish territory was occupied mainly by Russia and Prussia (later II Reich) until the end of I World War.
So first thing inheritated after this period (for 1919-39 years) is bad relations with Germany and Russia. After two polish uprisings (1830-31 - November Uprising; 1863-64 - January Uprising) Polish were to be culturaly submitted: "Rusification" and "Germanisation" proceeded. Polish emigration to France and USA also created very good relations with those countries.
Polish first years
Poland was created as an ally of Western Allies in XI.1918, especialy tied with France. In this time UK feared that France will be to strong, so it was against strengtening Poland. Especially UK's Prime Minister, Lloyd George and his foreing secretary George Curzon were against Polish gains in Greater Poland, Silesia, Bellarussia and Ukraine; they even refused to support Poland against Bolsheviks
In 1920 Polish Army defeated Bolsheviks (Miracle at the Vistula, VIII.1920) and ensured that "World Revolution" won't become true. In Riga Peace treaty (18.III.1921), however, Poland withdrawn itself from the concept of "federation" (Pilsudski idea - Poland allied with many independent/semi independent contries like Bellarussia, Ukraine, Lithuania, Latvia, Estonia, Finland and Romania), choosing Christian-Democrats option instead (big, national country - but with large minorities).
Democratic Poland was stabilised later on, with good relations with France (alliance 2.II.1921), Romania (alliance against Soviets, 2.III.1921) and Baltic countries, and bad with Germany, USSR, Czechoslovakia (Chechs broke an agreement with Poland and seized highly industrialized border area - Cieszyn Silesia) and Lithuania (Poland seized Lithuanian capital, Vilnus - it's population was mainly Polish but surrounding areas were mainly populated by Lithuanians)
Poland foreign relations 1926-35
In 1926, during crisis (Poland was economically very weak), Pilsudski led coup d'etat and introduced autoritarian government.
Relations with USSR
Poland knew what was happening in USSR - spies, smugglers and atrocities of the war with Bolsheviks (1920) helped to know Soviets better. That's why closer relations with them were never an option. However, as world economic crisis grown, Poland wanted to make better relations with them. Another reason was Soviet-German cooperation (dating back to Rapallo, 16.IV.1922) that could threaten Poland, should be stopped. Talks followed and Polish-USSR non-agression treaty was signed (25.VII.1932).
Relations with Germany..
..were much more difficult. Germany lost Greater Poland, Upper Silesia and Eastern Pommerania to Poland. Eastern Prussia was cut from Germany by Polish Territory. (It's not ideally shown in HoI2, as Gdansk [Danzig] was a Free City, populated by Germans but strongly tied to Poland, with control of the League of Nations).
German politicans claimed that Poland is a "Bastard of Versaille Treaty" and should be wiped out. After 1926, when Polish-German economic agreement expired, custom war started. However, Hitler regime wanted better relations with Poland, so Poland and Germany signed non-agression treaty (26.I.1934).
Altogether, Pilsudski choose and advised "Middle of the Road" option - not allying with Germany nor USSR, with close relations to western countries. But 12.V.1935 Pilsudski died of cancer..