In all this discussion of Poland's wisdom in suspecting German intentions, which was of course well founded, I am not seeing any discussion of the ramifications of Poland's participation in the dismemberment of Czechoslovakia, specifically the
Polish annexation of "Zaolzie" (aka Teschen) immediately following the Munich Accords of October 1938.
This clearly undermined Poland's moral position in the interim 11 months before the war decisively broke out with Hitler's attack on Poland. I daresay in terms of realpolitik it was a minor irritant versus the larger issues at stake, but I would say that if the Poles feared German intentions as much as they should have, they should not have compromised themselves with this opportunistic act. It is this that makes me suspect that the Poles may have led themselves to believe that they could cut deals with Hitler, and rejected his ultimatum because it demanded too much of Poland, perhaps even in the hope that with moderated demands, they could join the Axis against the Soviet Union.
I agree that Hitler would have to be a different person to honor such agreements in the long run, and that no matter how instrumental an allied Poland might be in aiding Germany against the Soviets, the day would surely come when Hitler turns on them and subjugates and destroys the legal existence of the Polish nation-state utterly, followed of course by ethnic cleansing leading toward genocide with a compromise of enslavement, while of course the Jewish population (some 30 percent of the nation of Poland in interwar years, IIRC) would be slated for total extermination.
But Poles, especially anti-Semitic reactionaries among them, might well convince themselves that Hitler would not be so unreasonable with a useful ally, and a broad spectrum of Poles would be keen to fight the Soviets.
So I'm afraid the notion that such short-sighted stupidity would be unthinkable, among the clique of reactionaries ruling the nation in the late '30s, seems to whitewash the Polish position more than is warranted.
And any possibility that Poland might regard the Germans as the strong power to align with undermines any certainty that Poland will come to the aid of the mutilated Czechoslovak state in March 1939. Remember that the Munich pact had not only stripped Czechoslovakia of the Sudetenland enclaves, which itself gutted the laboriously built up border defenses as well as leapfrogged the natural geographic terrain features they were based on, but also demanded of Czechoslovak authorities that they remove the existing leadership and put in a handpicked leader chosen by Hitler. There has been much discussion on this board of the ability of the Czechoslovak military to blunt a German attack in the fall of 1938, to deny him easy victory and perhaps even so stress the as yet weakly developed Wehrmacht as to discredit it before the world and possibly even provoke a military coup against Hitler. (The latter possibility I largely dismiss since I suppose the Nazi machine will be watching for it and would be pervasive enough to protect Hitler, put spokes in the wheel of military command legitimacy, rally Nazi loyalist and a certain number of the Army particularly recent recruits, and generally break the coup--but at any rate, a coup attempt would further cripple German military ability and encourage the French to attack Germany's western borders). In such a 1938 war, the Soviets could still do nothing without Romanian or Polish cooperation, but if they get the former, as seems more likely if the Reich is on the ropes militarily, then at least token Soviet forces having transited Romania to front lines in central Europe will be in at the kill, more likely than French or British troops would be to be on that front. With Germany tottering on the battlefield, battering against strong Czech defenses, Britain can not only send expeditionaries but also, with Danish acquiescence, pour the RN into the Baltic and give the entire German coast a heavy pounding. If the Danes see Hitler's control tottering ad are willing to stand aside for the RN, they might even be persuaded to join forces with British or French expeditionary forces, who would of course have to do the heavy lifting, and invade Germany from the north, with Denmark to be given the formerly Danish held lands annexed in the 1860s, or anyway the right to run a free plebiscite there giving the residents the choice between a rehabilitated Germany or coming under the Danish crown again.
So much hinges on how long the Czechoslovaks could maintain resistance, how long it would take German forces to be seriously worn down, which also depends on what the French and other Entente/League of Nations powers choose to do, bearing in mind also that the Austrian Anschluss earlier meant Bohemia itself was pretty close to encircled and would be attacked from the south as well as the north. If I had to bet, based on the knowledge I have right now, I would guess the most probable outcome would be the collapse of resistance before other powers made their decisions, in which case the Romanians and certainly the Danes would play it safe. A lot depends on whether this scenario is one where both Britain and France refused to be bullied by Hitler and indicated resolve to help the Czechs versus as OTL where they had already washed their hands of any responsibility. In the latter case clearly Hitler can concentrate more force and it would require an embarrassed change of heart in Paris and London to bring the beleaguered nation any hope.
However--this thread is now talking about 1939, when the loss of Sudetenland, annexed to German lands, and Teschen to Poland followed by massive annexation by Hungary in the south and east of Slovakia, as well as being politically semi-puppetized by German veto over the composition of the government are all realities. Even if we assumed that somewhat Czechoslovak fighting forces and morale remained undamaged, the loss of the fortified former borders puts them on a back foot, and the near certainty that the western Entente powers would do nothing for them demonstrated by the several outrages going beyond the agreement going unanswered all would clearly undermine resistance. Given the whole sweep of ugly reality, if a Czech premier or war minister were to order the forces to fight to defend the nation, there is every reason to think they'd disintegrate fast, and meanwhile of course German preparedness and equipment was leaps and bounds better in March '39 than in October '38. Crushing and assimilating Czechoslovakia will be a mere speed bump for Hitler by this late date; even if minded to Britain and France can do nothing to defend, only to avenge. They will get no cooperation from Denmark or Romania; the Soviets are frozen out.
It is not inconceivable that despite their greed regarding Teschen and guilt, Poland will at this late date recognize the writing on the wall, infer reasonably they are next, and jump in the Czechoslovak side while that nation still exists. Of course it would help in trying to justify that scenario to have a regime in Prague that is not already manifestly a German puppet! Since sadly this was the case, it is hard to see what could motivate any power, great or small, to step in and face the certainty of ongoing war with Germany, especially since geographically speaking few nations were in any position to help directly--in fact, it was down to Poland, and the Poles had already demonstrated contempt.
I don' think we can attribute the fact that Poland simply stood by in March 1939 and did nothing as evidence of solidarity with Czechoslovakia, which practically speaking had ceased to exist the autumn before anyway--rather, with the annexation of Teschen "Zaolzie," which did have a majority Polish population, the last plausible Polish claim on Czechoslovak lands had been satisfied. To go farther the Warsaw government would have to conflict with Germany with no particular grounds for further annexations than right of conquest. Hitler did not need or want their help after all. Hungary too left the remnant of Slovakia to Hitler's political management as an allied puppet state (which Poles would have cause to regret half a year later) presumably on the same combination of "we already have what we had strongest claim to" and "let's not stick our fingers in the mouth of this particular big dog at this particular moment when his bloodlust is up."
The Anglo-French Entente had washed its hands of Czech blood the previous autumn, there is no reason to think they'd put themselves at risk of war over the pathetic remnants they had stood aside to permit the ravaging of. Chamberlain indeed used the fact of German betrayal to steel Britain for war for the most principled of reasons coming up, but prior betrayals were water under the bridge. There would be no probable eruption of general war over Czechoslovakia's ultimate fate after Munich; the spark of general war would have to be over some other nation. As for Hitler forcing a Polish crisis at the same time as he mops up in Central Europe, that would not be smart for him to do, and no one is going to move on Germany preemptively. I don't believe Stalin would have if he could, and anyway the USSR is frozen out.
Only by removing Poland and or the Baltic states as buffers, either by shifting either to Hitler's side or Stalin's, or by one or the other moving in to invade and incorporate it, could Reich and Soviet forces meet face to face.