Yeah, but wasn't that inspired by a Japanese samurii drama?
It was, but it's really just one example. I mean, it's hard for me to see The Keep on the Borderlands as wholly unrelated to frontier forts, or PCs defending caravans from bandits as unrelated to all those troubles with wagon trains we see growing up. The old "stronghold phase" of high-level play was about carving out your own territory, and arguably some level of playing humanoids as not explicitly evil comes from looking back at "cowboys and Indians."
I don't think D&D is based on Westerns themselves, but I think it's very influenced by their spirit. Compare to, say, Warhammer Fantasy Roleplay and the old Lone Wolf books, which had a notably European voice.
But yes, samurai dramas have an awful lot in common with D&D themselves, given how easy they are to convert to Westerns. I think the Western's closer to classic D&D, though, where sensibilities are concerned. Classic D&D has more of a sense of class mobility, just like the Western, as well as tending to play out across larger geographical areas. In Seven Samurai, Katsushiro could never stay with the peasant girl; in The Magnificent Seven, Chico could do whatever the hell he wanted.
(The Magnificent Seven is also a picture-perfect ideal of really good players who give their characters unique motivations but buy into the adventure all the same, but that's a topic for another day.)
