The imperial attack immediately following the open scroll of "A New Hope" uses a chord that is note-for-note identical with the coda of "Mars" from Gustav Holst's The Planets. First, the Star Wars excerpt, lasting from 1:41-1:56 of the following clip:
Now compare that with 6:56-7:11 from "Mars":
For visual reference, here are the two score excerpts side-by-side:
Both pieces use the exact same dissonant chord: A-flat, D-flat, G from bottom to top, with a C in the bass. In Star Wars, for the last few measures Williams adds a C above the dissonant chord as well to keep the chord building in intensity.
Imperial Motif vs. "Mars"
Just as the Star Wars main theme shares similarities with the King's Row main theme, so too there are additional themes and motifs used in Star Wars that also bear similarities to historical predecessors. In this case, here is a comparison of the Imperial motif (not to be confused with the famous Imperial March that does not appear until "The Empire Strikes Back") with an excerpt of "Mars" from The Planets by Gustav Holst.
In both excertps the primary tone is G, with B-flat a clear secondary pitch. In the Imperial motif, the tertiary tone is A-flat, while in "Mars" the tertiary tone is F-sharp - both of which are a minor second away from the primary tone of G.
Much like the main theme's resemblance of King's Row, they are not completely identical, but they're undeniably similar in melodic contour, orchestration (Holt uses tenor tubas, Williams often but not always uses brass), and character.
The Music of Star Wars
These posts will help focus and develop my analyses of John Williams' film scores.