Here’s a direct quote from J. W. Rinzler’s “The Making of Star Wars”:
In enlarging the treatment to what became a nearly two-hundred-page rough draft, Lucas was continually aided by the transference of his Apocalypse Now ideas to the fantasy realm. Some of his notes scribbled on yellow legal pads are: “Theme: Aquilae is a small independent country like North Vietnam threatened by a neighbor or provincial rebellion, instigated by gangsters aided by empire. Fight to get rightful planet back. Half of system has been lost to gangsters … The empire is like America ten years from now, after gangsters assassinated the Emperor and were elevated to power in a rigged election … We are at a turning point: fascism or revolution.”
This is from when he was piecing together what the movie would be about, and is pre-rough draft, so around Spring 1974. You must remember that Lucas wanted to make Apocalypse Now after THX 1138, and the only reason he dropped it was because no studio would make it. But Lucas really wanted to make it, and while I won’t say Star Wars is Apocalypse Now in space, to deny Lucas’ politics and ideas at the time and how they influenced Star Wars is just incredible. It’s definitively kept as subtext - the best part of Star Wars is how it’s so many things at the same time right, not just a western, or just sci-fi, or just fantasy, etc. - but it’s undeniable subtext I think, especially when viewed through the lens Lucas provided with his prequels.