War comics in the 1960s largely shied away from Vietnam. It was a subject that I imagine editors thought was too controversial.
But Charlton Comics' war books did dip their toes into Asian waters from time to time. In 1967, Fightin' Marines #78 (cover-dated January 1968), we even got a regular feature about U.S. Marines fighting in the jungles of 'Nam.
I should say that this is the only story featuring these characters that I've read, so my comments will be based on that alone. If there was any evolution of the characters or overall theme, I simply don't know about it.
1. They have pretty cool names. "Shotgun Harker and the Chicken" has a definite ring to it.
2. The character dynamic reminds me a lot of DC's WWII-themed characters Gunner and Sarge.
Gunner and Sarge, by the way, had left the pages of Our Fighting Forces a few years earlier, but were soon to return as members of the commando team known as the Losers. Like Harker and Smith, this was a team featuring a tough, veteran sergeant and a young soldier who is very reluctant to fight but turns out to be really good at it. Gunner (like Chicken Smith) becomes his sergeant's go-to guy for dangerous missions.
Joe Gill is credited on Wikipedia as the creator of Harker and Smith, though the Grand Comics
Database has no writer credit for this premiere story. I have no idea if Gill or another writer was deliberately keying off Gunner and Sarge. It might have been a coincidence--the idea of a vet teaming with a newbie is, after all, a common one.
The story has nice art by Bill Montes and the plot gives us a good, straightforward war story. Harker and his men are fighting some Vietcong, who retreat down a tunnel. We immediately see that Harker is an aggressive warrior who effectively uses his shotgun. (By the way, my understanding is that a shotgun was indeed the weapon of choice for my U.S. soldiers and marines fighting in the thick jungle.) Smith is the reluctant one--when the enemy runs for it, he wants to let them go: "I'm no fanatic."
Harker initially pegs Smith as a coward, but soon realizes that, though Smith is a reluctant warrior, he is indeed a warrior. The enemy retreats down a tunnel too small to allow Harker to go after them. It's Chicken Smith who goes down the tunnel and smokes the Vietcong out with phosphorous grenades.
I love the panel you see above, where a clearly worried and surprised Harker looks down the tunnel after Smith has gone after the enemy. You can see his mind changing about his reluctant squad-mate.
So, when a mission to hunt down enemy rocket launchers comes up, Harker picks Smith as his partner.
The two have some adventures with North Vietnamese soldiers posing as South Vietnamese, I enjoy the moment when Harker realizes the soldiers are indeed the enemy because they don't wolf-whistle a passing girl.
In the end, they find the ammo stash for the rocket launchers and rig them to detonate in the tubes when the enemy attempts to fire them.
The character dynamic between Harker and Smith isn't original, but it's done well. Harker is written to be a little over-the-top ("I haven't enjoyed anything this much since my ex-girlfriend married a draft dodger back home!"), but this actually helps contrast him with Chicken Smith all the more effectively.
The feature ran through Fightin' Marines #108 (1973)--appropriately, this was the same year we pulled out of Vietnam. The first story simply ignored any controversy about the war to tell a character-driven war story. I'm going to steal someone else's comment from a comic book historians Facebook group to provide some more information on the series as a whole:
Next week, we go from waging war in the jungles of Vietnam to waging war on a desolate alien planet.