Weird Wars is a setting genre based on taking a real life historical war - World War 2 is a particular favorite - and making it "weird" by adding in super-technology, magic, or both. Note that this is distinct from fantasy or sci-fi war settings like Warhammer Fantasy Battles and Warhammer 40,000; the emphasis is on taking real wars from our world and making them fantastical, rather than just fighting wars in fantastical settings. This makes Weird Wars a more "rooted" genre compared to others like steampunk and its derivatives.
For those wondering just why World War 2 is a favorite for this setting, there's a couple of reasons. Firstly, the real-life experimentation in creating "wonder weapons" by the Nazis is an obvious inspiration to examine what would happen if you gave them access to full-blown dieselpunk technology. Secondly, the Nazis infamous interest in the occult makes giving them black magic another obvious thought experiment. There's a reason TVTropes has both "Stupid Jetpack Hitler" (Dieselpunk Nazis) and "Ghostapo" (Occult Nazis) as separate tropes. Thirdly, they're fucking NAZIS! You could not build a better human-shaped target to shoot lighting guns at guilt-free than them. Finally, it's all down to those pesky comicbooks; superheroes first debuted during WW2, and whilst there were some horror comics that had monsters taking part in the war, it's probably the fact that so many superheroes were depicted making their first debuts on the battlefields that really gave this genre a kickstart.
Aside from the actual game-lines named "Weird Wars", this is a setting genre applied to several different tabletop games, both roleplaying and war:
- GURPS has World War II sourcebooks, so of course it's got one devoted to adding fantastical elements to WW2. Appropriately enough, it's called "Gurps: Weird War II".
- Call of Cthulhu has both the classic Achtung Cthulhu!, the obligatory Lovecraftian WW2 book, and in more recent years a mini-line called Wold War Cthulhu, which focus on the great conflicts of the 20th century and how these interacted with the Cthulhu Mythos - currently, there's only The Darkest Hour (WW2) and Cold War, set during the height of the Cold War in the 1970s, but there are plans for WW1 and a fictitious WW3 sourcebook in that line as well.
- Deathworld is a /tg/-made weird wars setting that covers WW2, but also an alternate history up to 2011.
- Gear Krieg is an RPG and wargame set in WW2 where every major side in the conflict has access to armies of mecha and other dieselpunk wonders.
- Secrets of the Third Reich is a wargame set in an alternate WW2, where magic and super-science have kept the conflict going until 1949.
- Spirit of the Century, a Pulp setting for the Fate system, doesn't directly focus on this, but does discuss considerably the impact of Centurions (basically pulp-era super-heroes) on the First World War, and even provides rules for playing as the first generation Centurions during that conflict.
- DUST Tactics is a wargame with the standard Weird World War 2 package - mecha, powered armor, Nazi zombies, etcetera.
- Godlike is a Weird Wars superhero RPG taking place during WW2, with an expansion pack - Brave New World - detailing the Cold War.
- Deadlands doesn't directly focus on it very much, but the American Civil War is still raging with the aid of steampunk new weaponry, despite the various magical, demonic and undead horrors feeding on the soldiers from both sides. The Noir subsetting also mentions how World War 1 went, and Hell on Earth likewise discusses the Second and Third World Wars.
- Wolfenstein slowly develops into a Weird War setting as the games develop, as part of its ever-growing dieselpunk aesthetics.
- Konflikt-47 is a Weird Wars supplement for Bolt Action that adds dieselpunk vehicles along with Nazi zombies, vampires, werewolves, and werebears.
- "Rasputin Must Die!", part 5 of the Pathfinder Adventure Path Reign of Winter, sees a typical party of Pathfinder adventurers travel from Golarion to Russia, 1918, courtesy of the Dancing Hut of Baba Yaga, where they fight hypnotised Russian soldiers, nosferatu chaplains, Cossack dullahans, tanks animated by undead brains, sapient clouds of zombie-raising poison gas, daemons born from the souls of people killed in trench warfare, and Grigori Rasputin himself.
Weird Wars D20
Pinnacle Games, the makers of Deadlands, eventually bought into the D20 OGL enough to make Deadlands D20. But that wasn't enough for them; they wanted more. So, they decided to create an entirely new "weird history" setting, because, after all, it had worked so well with Deadlands. Weird War II was the fruits of their labor.
Released as the corebook "Blood on the Rhine" in 2001, it portrayed a world in which the Nazi atrocities had actually awoken ancient, magical forces, allowing magic to slowly seep back into the world. Player, of course, were Allied soldiers confronting the Nazis, the Italians and the Japanese, who had all become aware of the magic and were harnessing it. Literal German Wehrwolves, zombies, apes with Nazi brains, oni, ghost tanks and other monstrosities, all with nothing but grit, guns, and maybe a small assortment of useful magical spells.
Weird Wars II was quite productive; in addition to the corebook, there was a dedicated bestiary ("Horrors of Weird War Two"), sourcebooks for Africa & Egypt ("Afrika Korpse"), the Pacific Theatre ("Land of the Rising Dead"), and the Russian Front ("Hell Freezes Over"), a sourcebook specifically for aerial campaigns ("Dead From Above"), and an adventure splat ("Hell in the Hedgerows").
Savage Worlds: Weird Wars
When Pinnacle rebranded themselves as Savage Worlds, Weird War 2 went with them. And Savage Worlds decided to really exploit them. Aside from reprinting Weird War II, they also created three new lines of Weird Wars: Weird War 1, Tour of Darkness (Vietnam War) and Weird Wars Rome (Roman conquests). They even tied them together by using the same basic background info that they first created for Weird War II back in their days as Pinnacle; magic, especially black magic, gets stronger and more vibrant when humanity is experiencing active strife and conflict. So, when humans go to war, monsters wake up and start causing nuisances.