OK, let's forget Little Entente. Simply assume there is no Polish/Czechoslovakian debate about Zaolzie, Poland and Czechoslovakia have normal diplomatic relations, not too friendly, not too hostile. Both Prague and Warsaw look with growing nervousness at German armaments. Anschluss of Austria in March 1938 is a last call. When Hitler starts talking about Sudetenland, Poland and Czechoslovakia, with French blessing, sign mutual defense pact. Hitler is furious, but growing expenses force him to start a war or face economical disaster.
OTL Polish mathematicians broke earlier versions of Enigma code in 1932. In September 1938 Germans modified the code. Poles managed to break it a few months later. Then, in 1939 Germans modified their code again, and Polish cryptologists had not enough time to deal with it (basic method was the same, only they needed to make a lot of calculations - without computers). However, TTL in 1938, Wehrmacht and Luftwaffe still use code broken by Poles (Kriegsmarine already used modified version). Warsaw knows what enemy is planning and quickly warns Prague.
Hitler decides to start war in spring 1939, His first target is Poland - he believes that after defeating Poles Czechoslovakia will surrender. However, warned by Polish intelligence, both country mobilize their forces in time. Poles don't sell all their best fighters (PZL P-24) abroad, they also keep more anti-aircraft cannons 40mm Bofors (insted of selling them to...Britain).
So - in 1939 Hitler doesn't have at least 2 armoured divisions he had IOTL. His infantry is also weaker. What is worse, even if he sends main forces against Poland, he must keep some division to keep Czechoslovakian in check AND keep some divisions in west - what if France decides to intervene after all, even without British support?
When the war starts, Poles quickly learn to respect German Panzertruppen. OTOH, for Wehrmacht it isn't easy either. Every time their tanks meet entrenched Poles, casualties are enormous. While German mechanized columns push towards Warsaw, German infantry must deal with Polish forces - and German advantage is much, much smaller than IOTL. When the main German strike reaches outskirts of Warsaw, on their flanks fall Polish counterstrikes. General Kutrzeba's Army "Great Poland" attacks from north, Army "Kielce" with Polish only motorized brigade, reinforced by the remains of Army "Pilica" (defeated, but not destroyed by the first German attack) attacks from south. In the battle of Kutno Poles get get support of 2 Czechoslovakian fast divisions (cavalry/mechanized forces). Thanks to brave raids of Polish and Czechoslovakian fast units, German spearhead is cut off from supplies - especially fuel. Elite Panzertruppen became suddenly badly trained infantry.
In north Polish army "Pomerania" is defeated and pushed south, but not destroyed. However, Germans control Pomeranian corridor, with Polish Coastal Defense desperately fighting to the last shell. Army "Pomerania", reinforced by some reserves, is still able to cover army "Great Poland", while Kutrzeba keeps hammering von Runstedt's northern flank. Finally, German forces advancing to Warsaw are pushed buck, and one too adventurous armoured division is surrounded in Polish woods and forced to surrender.
Polish Navy after heroic, but short fight, is either destroyed or interned in Sweden. However, Poles managed to lay mines near Danzig port, which costs Germans some smaller ships and a lot of headache. Meanwhile in Gdynia and Hel Polish forces keep fighting.
Luftwaffe didn't manage to destroy Polish air force on its arfield (that didn't happen IOTL either), but attacked strategical targets in Poland and Czechoslovakia. German pilot suffered serious casualties, especially over Prague, when they met with Czechoslovakian fighter planes (better than Polish). As a revanche for bombing Polish and Czechoslovakian towns, Polish Bomber Brigade attacks targets in German Silesia, then lands in Czechoslovakia. Next evening Poles start from Czechoslovakian airfields, bomb Germany again and come home.
German minorities in Poland and Czechoslovakia either refuse to fight or actively hel Wehrmacht by sabotaging war effort of Poles and Czechoslovakians. Prague and Warsaw react quickly - most Germans in Silesia, Great Poland and Sudetenland are interned. There are some casualties, even atrocities.
Hitler asks Hungary to attack Czechoslovakia, promising Horthy pretty much anything he wants. But then Romania, backed by France, warns Budapest, that if they try to interfere, Romania will do it too. Romania doesn't act because of friendship towards Poland or Czechoslovakia. They simply are affraid that Hungarians demand from Hitler Transylvania as payment for their help. Besides, Hungarians aren't particularly interested in fighting Poles - traditional Polish-Hungarian firendship is still very strong.
So, after a month of fighting Hitler is not in good mood. Blitzkrieg didn't exactly work. Poles lost a lot of soldiers and equipment, but their losses are mostly infantry and cavalry, relatively easy to replace - especially with Czechoslovakian industry working all around the clock. Polish army is still able to fight. Germans, however, lost a lot of their tanks and tank crews.
Germany controlls Polish Corridor, western part of Great Poland and northern Masovia. Polish Silesia still resists, so does Polish Coastal Defence. Hitler publicly declares he is ready to make peace with Poland and Czechoslovakia, if he is allowed to keep most of already occupied territories. And Sudetenland. Despite British pression, Poland and Czechoslovakia refuse.
Meanwhile France wathces the war and decides, that after all Germans aren't so tough, and with most of their forces engaged in east, they very easy target. So France declares, that they will not allow their traditional (more or less) allies fight alone and start mobilization. German generals are terrified and decide to act. Organized under leadership of Canaris they overthrow Hitler and propose a cease-fire and then return to status quo ante bellum (pre-war status). Poland and Czechoslovakia agree, but they also demand compensation for their losses - some of it can be paid in military equipment. As minister Beck explains to German negotiators - "you won't need it anymore, will you?". With French forces on their western border and stallmate situation in east, Germans have to agree.
OK, I'm Polish, so it is a little Polonowank, even if not so unealistic. Oh, and the conduct of war is not toally mine - I borrowed a few ideas from an article by Wojciech Zalewski in "Taktyka i Strategia" about "Autumn War" - when POland defeats Germany in 1939....