Nothing unites better than a common enemy. Problems? Olsa or Tesin Silesia, if you prefer. Both sides wanted to take it, there supposed to be a plebiscite, but when Poland was busy fighting Soviets, Czechoslovakian army took it (summer 1920, I believe). Also Poles was a little angry, since Czechoslovakia sometimes blocked transports with supplies for Polish Army.
How to change it?
Germany, March 13th, 1920. Right wing extremists in Berlin organize a coup d'etat, proclaiming Wolfgang Kapp the Chancellor of Germany. Republican geverment flees to Stuttgart, but its train has an accident. Not everybody dies, but republicans are paralized for a few days. The communists start uprisings in Ruhr Region. So starts German civil war.
Western powers are unsure what to do: they don't like the German righ-wing , but they scared of communists. When Soviet army reaches Warsaw, France and Britain give logistical support to Kapp to prevent communist victory. In 1921 Kapp has Germany under his control. He swears he has no territorial claims in the West, but doesn't mention a word about German eastern borders. He is allowed to maintain bigger army to keep the communists at bay. After Kapp's death in 1922 his closest follower, general von Luttwiz, becomes new Chancellor. He manages to surpress hyperinflation and pay war reparations. In 1925 he starts talking about reclaiming "stolen" lands from Poland and worrying about "surpressed" Germans in Czechoslovakia. He starts a rearmament program, although not at such scale as Hitler in OTL. It is enough, however, to make Poland and Czechoslovakia nervous.
In 1930 German citizens of Danzig organize a pustch and demand return of that town to Germany. Luttwitz swears he had nothing to do with this, but he has to protect his countrymen. He also guarantees Poland special privileges in Danzig port - IF it comes back to Germany.
France and Britain don't want war. They organize a conference about Danzig. German diplomats manage to prolong it, while German and Polish armies stay ready to fight. Unfortunately, League of Nations wants to let people of Danzig decide: since Germans are majority there, Danzig is German again. Poland is furious, but France and Britain warn Polish goverment: if Poland tries to fight for Danzig, it will fight alone.
Meanwhile, Luttwitz starts great propaganda campaign to "let people decide whose citizens they want to be". Add to it Great Crisis and Poland gives up...for now. But Marshal Piłsudski is not happy and starts secret talks with Czechoslovakia.
Prague is also worried, since they believe that Germans in Sudetenland may try something similar to "Danzig uprising". Finally, on 4th of April 1932, Poland and Czechoslovakia sign mutual defence pact. Two weeks later they are joined by Romania, some time later by Latvia and Estonia. Poland officially gives up any claims to Olsa. The military pact is followed by economical treaty, so all interested countries try to deal with Great Crisis together.