It sounds like you are conflating two different things: the intelligence of a population and the intelligence of a ruler. The intelligence of individual commanders absolutely does matter and has been written about for a long time. Hannibal Barca is famous for consistently outsmarting the Romans. The Battle of Cannae in particular is famous as a case study in one commander outsmarting another.
As for populations, the intelligence of any population (with some exceptions) suffers a regression to the mean, which refers to how large populations are less exceptional than small populations. An individual commander can be much smarter than another commander, but nation-sized populations tend to vary little on traits like intelligence due to the Law of Large Numbers. Differences between populations' intelligence is swamped by other factors like leadership and industrialization, which do vary between nations.
Until recently, national success has been attributed to the superiority (including that of intelligence) of one population over another. These arguments have been followed by genocide and other atrocities so consistently that the whole argument has become tainted by association.
I think the issue boils down to only one thing. Average national intelligence is irrelevant. If the nation has a system that allows and selects for the top few % to be identified and educated, the nation will do well. The rest of the population are just lever pullers and button mashers in the big picture. The handful of brilliant engineers designing weapons will allow the nation's armies to be victorious. The handful of brilliant economists and businessmen will allow the nation's economy to grow most successfully. The handful of brilliant.... you get the idea.
If two populations are statistically identical and you select the top 1% of one population and put them in positions of influence and power, but you select randomly from the top 10% of the other population and put them in the same positions, the results will be dramatically different. On some small level, average intelligence might matter, but the outliers matter vastly more. Energy spent identifying and promoting the outliers will always be energy well spent.
I think your question is just based on a wrong premise. I have read quite a bit of World War II history as well asSome history of the Vietnam war and the decisions of Hitler and McNamara in particular are often described as mistakes clearly based on their intellectual ability, cognitive bias, and misconceptions. People don’t necessarily say “this means Hitler was stupid” but the implication that he would’ve done things differently if he were smarter are obvious. Moreover Churchill is depicted as prescient in his assessment of Germany’s rearmament. The words unintelligent and the like are basically just left to the reader to infer.
You ask a bunch of questions so I’ll try and break them apart and take them in order.
There are scholars who believe that the average intelligence of a nation in packs outcomes such as GDP growth. See the work of Garrett Jones. https://scholar.google.com/citations?user=xXXJZ-MAAAAJ&hl=en
Intelligence is often left out of models for many historical institutional outcomes partly because academics see it as a info hazard. This should bias any regression analysis due to omitted variable bias. For example a institutional quality is correlated with group intelligence then the estimator for the impact of institutional quality is biased by the correlation factor times the impact of intelligence. Therefore there’s a price for avoiding that info hazard.
Measurement bias is a problem. It can be imperfectly corrected for with wordless it tests. Differences between countries in average IQ remain under these tests, see Jones.
For individual leader quality differences in group intelligence shouldn’t really matter. Yes some leaders are stupid.
The relationship between the mean score on individual iq tests of a country and the collective intelligence (have 4 random people take an IQ test together) is a different question. idk the research on it.
So... what I have often wondered when reading about historical events is that it is very rare to read a story where it is asserted that historical actor A was victorious over actor B due to actor B being a bit of a dumbass, or actor A being smarter.
In fact, one could be forgiven for thinking that this is some sort of taboo; the outcomes of wars might be down to various sins such as pride, ill-advised beliefs about ones superiority or ascribed to the decisive tactics used, but almost no action is ever being attributed directly to cleverness, or any loss to thoughtlessness or just being dumb. Now one might argue that, say, the tactics of an army are the outcomes of intelligent thought, and of course this is true to some extent. But I am just curious why it is that history books in general basically appear to work on the implicit assumption that each nation being discussed has the same level of intelligence and the whole concept has no mentionable role to play in the run of history and the events as they transpire.
And this is very odd indeed, because it would seem to me that the vast majority of outcomes of a given interaction between nations, rulers and populations depend to a critical degree on the intelligence of the parties involved. Yet, this aspect appears to be hardly ever discussed. You'd sooner read about the ill health of a ruler, or the kind of extravagant palaces he built... intellectual capacity might also be mentioned, though if so, often as a general (inferred) trait, without elaborating on its impact in actions taken.
So why is that? Perhaps it is too hard to measure or ascertain...? Easier to count the number of cannons than the "IQ" of the generals...? I'm curious what you will say :)