Suppose someone was willing to give a modern air force a bunch of
planes from WWII. Are there any WWII aircraft that could still
be useful in a modern war?

This response assumes that by "modern war" you mean a fight against an
enemy who knows he's at war and has meaningful radar-based
surface-to-air and air-to-air assets. The former criterion lets out
sucker-punching an undefended target in terrorist/insurgent fashion
(postmodern war?). The latter criterion excludes third-world
uglinesses whose technological level doesn't extend to SAMs and modern
AAA and jet fighters.


Period fighters would be pretty useless against modern ones except in
"golden BB" scenarios. They might be able to evade and survive, but
merely surviving isn't accomplishing a mission. With their energy,
armament, and sensor and guidance systems based on slight
augmentation of the Mk I Eyeball, it's hard to imagine what they'd
have to say either offensively or defensively to an intelligently
flown jet. ("Intelligently flown"? Well, I suppose that if the jet
pilot were silly enough to play to their strengths while ignoring his
own...) If the jet were pantsed by circumstance (taken unawares in a
low energy state, or running on the memory of where the fuel used to
be and thus unable to fight, or whatnot), that too would work, but
overall one may bet heavily even on early jets.


Something to game on a rainy day would be a saturation attack on a
point target such as a CV battle group using large numbers of
attack-capable fighters, torpedo bombers, or dive bombers. You might
score, though probably at terrible cost. How you would marshal a
suitably large force undetected is another question entirely, and
that's probably what puts it into the realm of paperbacks.


Heavy bombers of the period trying to drop iron bombs over defended
enemy territory today -- sounds like a silly waste of men and
machines even if escorted by period fighters. They might be useful
as ALCM trucks if they could launch before being intercepted. Some of
them could certainly still give yeomanlike service in antisubmarine
warfare if granted modern sensors and weapons, or as ELINT or AEW
platforms.


Many of the twin-engine cargo planes could play a role in theater
airlift if nothing more modern were available, and could drop
paratroopers.

Strategic airlift in those days was a technology-limited embryonic
notion, with converted bombers hauling small amounts of stuff at the
speed of a mortified tortoise. Aircraft that were follow-ons of WWII
aircraft were successfully used as tankers for jet fighters, but it
was a hairy enough proposition that the advent of the KC-135 was a big
improvement.


All in all, there are reasons jets and turboshafts elbowed their
recip/prop predecessors into niche combat roles, and ultimately into
the surplus market, as their capabilities improved. Their speed and
power let you do more and have better odds of surviving the attempt.


but you still have to maintain them?

Ah, there's yet another question: whether these aircraft (no longer
widely familiar, and not always easy to operate or to maintain) come
out of the time warp with their aircrew, ground crew, spares, and a
lot of high octane?

Cheers,
--Joe