"[U]naware of the opposing air force's knowledge of the chaff concept, planners felt that using it was even more dangerous than not, since, as soon as it was used, the enemy could easily duplicate it and use it against them... for over a year the curious situation arose where both sides of the conflict knew how to use chaff to jam the other side's radar, but refrained from doing so fearing that if they did so the other side would 'learn the trick' and use it against themselves."
Using a modern missile against an indigenous people will only impart that you are capable of that type of attach.
Using a modern missile against WWII Germany would likely quickly result in refinements to their V2 Rocket program, given enough remains to study.
Using a modern missile against Vietnam era USA would likely result in advancements in miniaturization and computation, given enough remains (even if they did not have the resources/facilities to capitalize on some aspects of those for years, I think it's likely it would advance the fields by a least a few years).
One of the biggest advantages the Allies had in WWII was that they had cracked the "uncrackable" Axis encryption. Even though they were able to decipher enemy messages, they often didn't act on that information because that would tip their hand. The strategic value of reading the enemies messages is enormous when the enemy doesn't know you can do it, and much less so, and possibly even negative when they do know.
It's also along the lines of Sun Tzu-esque deception.
I suppose the modern example are the constant probing of air defenses by the attacker (i.e. the US and its array of electronic warfare suites), and the game theoretic calculation by the defender on whether to turn on their radars or not...
IIRC the proximity fuses were developed at the Johns Hopkins University Applied Physics Laboratory (JHU/APL); that is the story I got when I worked there.
IIRC, the shells were also especially effective as anti-aircraft artillery.
It does. WWII tanks' armor is mostly concentrated to the front and sides, because those tanks are designed to force enemy lines against ground-bursting shells, field pieces, and other tanks, all of which fire mostly on low trajectories; what's on top is much thinner, because no one expects to need to withstand a lot of damage there. Bursting a shell above ground level throws a lot of fragments at that weak armor, where a ground burst mostly wastes them against armor designed to withstand direct hits from much more powerful weapons. For infantry, it's even worse; the whole point of a trench or a foxhole is to put a thick layer of earth between you and all the metal that's flying around at ground level. When an airburst can send fragments right down into the hole with you, that earth doesn't help one bit.
Fun fact: "daisy cutter" bombs work the same way. Up until Vietnam at least, their proximity fuse was on the end of a rod protruding a few feet from the nose of the bomb. Low-tech compared to a radar proximity fuse, but fearsomely effective; probably the only reason you wouldn't find it on a shell is that, unlike an air-dropped bomb, a shell has to withstand the force of being fired from a gun, and I doubt any such expedient could. (That's also why bombs tend to be so much more effective than shells, even when no more accurate. When the strongest force involved is 1g, you can spend a lot less mass on structure, and a lot more on explosive.)
The sight is arranged so that if you aim at the tank, the weapon is actually aiming above it. Then the round will detonate as it pass over the target, sending a molten metal shaped charge right down.