My Martian alternate history has a Martian attack every 20 years, starting in 1898. Each attack takes place in a stand alone universe, so each attack is a unique and unprecedented event.
The first attack is like that of the H.G. Wells War of the Worlds book. The Martians have tripod walkers, heat ray, black gas, and red weed. They have limited vulnerability to Earth weapons.
Then there is a 1918 Martian attack. I use the same Martian tripods but the humans get World War One era weapons. It makes it a little more of a challenge for the Martians. This is similar to All Quiet on the Martian Front, but I use mostly historical vehicles and equipment for the humans.
In 1938 the Martian attack is not unlike that of the Orson Wells radio broadcast. It gives me a chance to use those not quite World War Two US Army vehicles and troops, mixed in with some of the World War One stuff. I add an anti-Meson weapon that disintegrates things rather than just set them on fire or burn them up.
An attack in 1958 by the Martians is along the lines of the War of the Worlds, George Pal movie from 1953. The Martians get the boomerang war machines and force fields. A very tough nut to crack.
So far I have not worked out anything beyond that. Each group also gets infantry, I use Mars Attacks type figures as the infantry.
There is much more but this is a brief overview. I would like to thank some of those TMPers who helped with comments when I asked questions here.
They include but are not limited to: darthfozzywid, jpattern2, Olaf the hairy, Bob without his dog, Stryderg, skippy0001, Major Mike, alien BLOODY HELL surfer, Dances With Words, Iasalle012, Ghostrunner, Mobius, chironex, 28mmMan and no doubt others; thank you all for the help.
Mike Bunkermeister Creek
bunkermeister.blogspot.com