I don't know what the right action would be, but the action taken -- essentially do nothing -- was clearly the wrong move and led to Germany taking over Western Europe for several years.
What was the right move then? Please detail. Would it be an “attaque à outrance” with an unready French army and an embryonic BEF against fortified German positions, with neatly inferior air forces, not enough siege artillery to break the Siegried line, a flimsy logistic branch, and following a totally hurried plan due to an uncooperative Belgium which screwed up all pre-war planning? And all that within a nation which already suffered humongous and material human losses barely 20 years before and could not really afford the same thing again, neither from a political nor a practical perspective if it was to handle a long war.
No, I argue that even with hindsight, globally, the right call was made: there was absolutely no way to save a Poland which happily sacrificed every opportunity to get military allies less than 1,500km away during the whole interwar period; the only playable hand was to bet on a long, tracted war where the French and the British could economically strangle Germany like they did in 1918 and free Poland afterwards – implicating, on the ground, turtling behind the border defenses long enough for the blockade to do its job. Problem: (i) Belgium royally screwing up the plan, (ii) USSR joining the waltz, (iii) the incredibly lucky strike of the Germans in the Ardennes.
Sometimes, the only winning move is not to play.
Now I totally understand why they were not fond of FR/UK deliberately planning to sacrifice half their country to establish strong defensive lines on their rivers, but sometimes you can't have it all and just have to go with the less shitty plan.
I’d imagine that the literal millions of dead under Belgian soil, and the total destruction wrought by WWI made them hesitant to take any sides in potential conflict. You don’t position yourself in the middle a fight between two heavyweight boxers when you are a flyweight.