I like to treat the premise with all due seriousness, and then sprinkle it with sly humor and whimsy. I mean, heck, anyone who looks at my Free Norway Project knows I like to build backgrounds that explain what this ridiculous combination in front of you is, and how it came to be...
US Cavalry on a D&D bulette - oh, well, the Lennsmann were trained by US Cavalry officers, and the bulette is a form of sandworm that evolved in the highlands, where there is less sand and more solid rock...and then I'll have them led by a man named Captain Armstrong, an elderly American soldier who fled earth after a failed military sorty, and is thought to be dead by his former associates.
And name my desert Martians' religious leader Mullah Dib.