In the classroom, it was not a "if you're gonna invade Russia, bring winter clothing" kinda thing.
And there's a reason why we discourage comparisons to "lessons for the present day": they muddy the discussion rather than clarify it, and the reason they muddy the discussion is they assume that history is a collection of facts about the past.
It isn't. History is made by producing arguments based on evidence from the past. We can't just assert something happened, we have to show that it did, and the data does not easily admit of a single, global understanding.
So, contrary to what Hap may think, there's plenty of good and exciting history to be done: as our understanding of the world changes, so does our history.
Bringing comparisons to the present in the classroom assumes history is static, that "we know what happened", so there's no point in investigating it. If you want to put it schematically, the difference is:
History:
Evidence --> Interpretation
Rhetoric:
Interpretation --> Position on Current Events
The moment you inject the present into an interpretation of the past, it raises all kinda of red flags: is your argument based on a fair assessment of the evidence, or merely what you want to see?
Moreover, as your "winter clothes lesson" clearly demonstrates, seeking present comparisons leads to the single-factor fallacy. Very few major events in human history have a single cause that is necessary and sufficient. Did the Germans lose the war because they didn't bring trenchcoats to the Eastern Front? Or were there deeper logistic problems that might have also contributed?
Ultimately, what really gets us annoyed with the "past/present" comparison, is that so many people are content with what they think happened in the past, that they don't care to know the truth. For this reason, I've taught University history classes by starting out with the book Inventing the Flat Earth:
Basically, the story is the following: a surprising number of people believe that a main purpose behind Columbus' 1492 voyage was to prove that the Earth was spherical, not flat, as the Church insisted everyone believe. Even a Pulitzer-prize winning former Librarian of Congress wrote a popular history book where he argued that Columbus was a brave man to stand in the face of Church-inspired ignorance.
The whole story is a lie, a huge bit of horsecrap, that bears only a negative relation to what people believed at the time. By negative relation, I mean they knew the world was spherical, and in fact, everyone with any sort of education in the past 2500 years of the Western Tradition took for granted that the world was spherical. Only a few crackpots believed the world was flat, and none of them had any roles of significance in the Church hierarchy. Incidentally, to make a past/present comparison, there are tons more of those flat-earth crackpots around today than there ever were.
How did this outright lie make it into our popular culture, and even our history books (presumably the ones Hap is talking about, where all the good stuff has been said)?
Why, of course, because certain people wanted to use history as ammunition for current debates.
In the late Nineteenth-Century, in the wake of Darwin, in the US we had something that secular progressivists liked to call "the war between science and religion". They believed in a bunch of silly notions, chief among them were that scientific and moral progress go hand-in-hand, and that religion is naturally opposed to scientific progress. So, seeing the opposition of various religious groups to Darwin's theories, they hunted and found a fictional account (by Washington Irving) of Christopher Columbus' life with this myth in it, and used it as ammunition: "See, this is what happens when religion rules science". They put it in geography textbooks, and the idea took off, not because it had any relation to reality, but because it fit what people wanted to believe was the case.
So as a result of this, millions of people believe that educated medieval churchmen thought the world was flat, an outright lie.
And that is why we discourage comparisons to the modern world: focus on the evidence, and find the story.
If there is an old history, it would be: "Here are the facts about the past. Memorize them and repeat them on the test." And the new history would be: "Here's the evidence from the past that we have, here's how we interpret it, and here are our results. Now you try it."
