I'm not sure that Britain provided a formal guarantee of Poland's security.
They did; it was signed on 25 Aug 1939.
My reading of the agreement is that Britain agreed to do "all in their power" in the event of Poland being attacked by an unnamed European power. Note the actual guarantee was for Poland's independence and not its territorial integrity. Again, it comes back to the question of exactly what Britain could have done that was "in their power"?
This is hair-splitting similar to that which gave rise to the inaction of autumn 1939 in the first place. Their signed agreement was for each to aid the other operationally in the event of an aggression by a third party.
As for what UK might have done, perhaps send its navy to offensive action against German ports? In any event, if it could do nothing at all, perhaps the UK should not have given out a mutual defense pact. If they truly were impotent, they committed their nation to a war they could not fight.
Sending the RN into the Baltic isn't going to stop the Wehrmacht rolling all over the Polish countryside. Given the size and concentration of the Kriegsmarine's U-boat fleet, such a move would make the RN vessels more of a target than a threat.
My interlocutor was implying that the US has the means but not the will to save Poland. It is, however, inarguable that the UK had assumed the legal obligation to come to her aid.
As for Czechoslovakia, that state had only been in existence for 20 years. I don't recall much of a bruhaha in recent years when Sudan split in two. Why would the global powers care overly about a state that was about a third of the age of most politicians of the time?
That doesn't excuse the miserable sell-out. Or, if it does, at what age does a nation ascertain its right to national survival? Three decades? Five? Ten?
As for why they should care, France in particular was reliant upon the Little Entente to counterbalance a stronger Germany. Czechoslovakia was a linchpin in this plan because 1) it had a strong and competent army; 2) it had great fortifications in (wait for it --) Sudetenland; and three, in the Skoda works, it had its own source of weaponry and was less-reliant upon Western equipment.
So yeah, they should have cared. And they did, only not so much as they cared for avoiding war.
I hate to sound callous but it's not like there was a long history for Czechoslovakia as an independent nation, indeed the country itself decided to split up in 1992 which strongly suggests there was never much in the way of glue holding the nation together.
That's aside my point of bringing them up in the first place. My point is that it was foolish of the Poles to trust any pact signed with UK or France when those two countries had permitted the destruction of another country formally allied with them.
Rhe alliances between France and Poland, and Britain and Poland, were inherently defensive in nature. Suggesting that such defensive alliances should have prompted France to invade Germany seems rather optimistic. It would be rather like expecting NATO to invade Russia because of the latter's attack on Ukraine.
You don't seem to understand. France was committed to take offensive action in the event of the Germans invading Poland.
It should be noted as well that NATO had no signed agreement to defend Ukraine, while the UK did with Poland before Germany invaded. Thus it's a moot comparison.
There have been lots of posts that Britain and France should have done "something" but I've yet to see a tangible recommendation for what that "something" should be.
How about pushing the Saar offensive forward with vigor, instead of stopping after ten miles? The Germans, literally, had twelve divisions guarding all of the western border. All of it. The French had 100 divisions on the same frontier, and further, were treaty-bound to attack Germany in the event the latter attacked Poland. The French shirked the obvious riposte.
As for the UK, they did what they could at sea, but spent the next nine months dropping paper.