International relations involves competing aspects of structure and agency. Sometimes structure prevents you from doing what you would wish to do and geography is part of that structure problem, just as it was for the USA. The US may have wanted peace in the world but it was geographically isolated and, in 1939, lacked the political will and military teeth to do anything. Britain and France hoped that a formal threat of war would dissuade Hitler. He called their bluff...but they followed through on their promise. That doesn't make their promises vacuous. It simply means there were limits on their ability to act.
Would you prefer that Britain and France gave Germany free rein in Poland without threatening to go to war? It could be easily argued that it was not in Britain's best interest to go to war in 1939. Britain was under no direct threat of attack from Germany. Avoiding war would have preserved the British Empire, continued trade, and saved hundreds of thousands of British lives, both military and civilian.
It's not a vacuous promise when it leads to the deaths of 384,000 soldiers and over 70,000 civilians from your population for a war which, arguably, wasn't in your national best interests. That's putting real, physical skin (and blood and bone) in the game.
Without disparaging those sacrifices at all, didn't the British leadership know that it wasn't able to take action? And what does that say about those promises?
As for what I wish they'd done instead, I would have liked to see both nations abandon the policy of appeasement much earlier ... before Germany got so powerful they were rendered impotent.
And I will now definitively leave off this derail. If you wish to discuss it further, open that thread, I'll happily partake.