"Virtually nobody on "the left" will ever claim that France and the UK should have not declared war on Germany when it invaded Poland so as to decrease suffering."

For this I have the following possible explanations:

- They supported a war against Germany not for the protection of Poland but in order to topple the fascist government and stop the atrocities that were being committed.

- They supported an early war against Germany because they believe that the German government would keep going with their demands and that the war was inevitable.

- They are against the war but admitting it would be a social suicide.

- They don't have an opinion about it or just have not voiced an opinion about it. (as it is an event that happened decades ago instead of one that is happening at this moment)

"funnily enough, whenever I bring this point up with those who claim the West is "prolonging suffering" by supporting Ukraine, they never concede that France and UK's decision at the time would be consistent with their claimed non-interventionism, nor that the USSR prolonged suffering by supporting the eventually defeated Spanish Republic."

This is interesting. What is their usual counterargument? I Presume that they either don't have a consistent logic behind it or they believe that protecting Ukraine would not benefit the common people (or would benefit them marginally) in comparison to the Spanish civil war which in their mind supporting the republican side would benefit the common people enough to counterbalance the loss of lives.

> > "funnily enough, whenever I bring this point up with those who claim the West is "prolonging suffering" by supporting Ukraine, they never concede that France and UK's decision at the time would be consistent with their claimed non-interventionism, nor that the USSR prolonged suffering by supporting the eventually defeated Spanish Republic."

> > "What is their usual counterargument?"

One that stuck to my mind was, believe it or not, that an internal armed coup against a democratic government does carry the moral obligation of other democracies to intervene and defend it if said government requests it—which is not the case apparently when a democratic government requests support to defend itself from a neighbouring state which invades it without the slightest provocation. It's basically all post-hoc rationalisations of their emotional beliefs.

> For this I have the following possible explanations:

> - They supported a war against Germany not for the protection of Poland but in order to topple the fascist government and stop the atrocities that were being committed.

What on Earth are you talking about? France and the UK did not come to Poland's help in September 1939 not due to an altruist aspiration to stop Nazi atrocities that had not yet begun, but due to a mere fact that neither country was ready for an all out war with a heavily industrialised Nazi war machine.

Nazis invaded Poland on the 1939-09-01, but the first ever death camp at Bełżec in Poland had not been built and had not become operational until March 1942 (Operation Reinhard) with more to follow over 1942-44. The truth about the nature and the scale of Nazi atrocities had not surfaced to Western Allies until 1943 which was well into the WWII, and when it did surface, the Western Allies at first refused to believe in reports due to the unfathomable and previously unseen scale of the atrocities.

According to the official Nazi racial doctrine, all Slavic peoples (Poles, Czechs, Serbs, Ukrainians, Belorussians, Russians – all of the Slavs, indiscriminately) – with Poles specifically – were subject to a complete and indiscriminate extermination; from Himmler's speech on 1940-03-15 in Poznan:

  Himmler spricht in Poznan vor den versammelten Kommandanten der Konzentrationslager. Eine seiner Aussagen: "Alle polnischen Facharbeiter werden in unserer Rüstungsindustrie eingesetzt. Später werden alle Polen aus dieser Welt verschwinden. Es ist erforderlich, dass das großdeutsche Volk die Vernichtung sämtlicher Polen als seine Hauptaufgabe versteht.".

"…das großdeutsche Volk die Vernichtung sämtlicher Polen als seine Hauptaufgabe versteht" is key here.

The worst was yet to begin in 1942, not in 1939.

This is irrelevant. We are talking about current leftists with our current knowledge of history.

But France in WW2 did just that - it failed fast its territory over to Germany rather than going the trench warfare and scorched earth path.

"Failing fast" was hardly an intentional decision on the part of France. In fact, one of the principal reasons why Germany was able to so quickly overwhelm France was precisely because France was still preparing for the last war, i.e. trench warfare. Related to that, after the fall of France, the British Cabinet was intensely split between those on the side of Churchill who believed that the UK should "never surrender", and those on the side of foreign secretary Lord Halifax (and initially Chamberlain) who believed that the UK should offer peace terms with Germany to avoid a crushing defeat [0]. Had Chamberlain not changed his mind and Halifax's view had prevailed, one could argue that the UK would have chosen to "fail fast" and thus avoided the Blitz and much further suffering of its people—but at what cost?

[0] https://en.wikipedia.org/wiki/War_cabinet_crisis,_May_1940

Providing weapons is a major intervention. That’s what the US did in the war between Pakistan and Bangladesh, and they ended up supporting a genocide.

You can try to rationalize a distinction by saying in that case, the US failed to support the people seeking independence. But then you have to be in the business of deciding which people’s claim of independence is legitimate. Supporting the jihadists in Afghanistan against the Soviet Union didn’t exactly end up well. Intervention is always rationalized in the moment.

The jihadist claim of independence against the Soviet Union was, if nothing else, entirely legitimate.

(Preemptively: you do not under any circumstances "gotta hand it to them", etc, etc).