While the melody of the Throne Room is the Force theme, the introduction bears strong similarities to the introduction of the famous "Wedding March" from A Midsummer Night's Dream by Felix Mendelssohn.
To visually articulate the similarities, here are two score fragments, with the Throne Room intro on top and the Wedding March intro on bottom.
Both start with trumpets (The Throne Room also uses horns) on middle C with a fanfare-like rhythm (indicated in red), followed by an expansion of that initial motif to include the E above middle C (indicated in green). The Throne Room also adds the A-flat below middle C. The motif is further expanded to include the G above that (indicated in blue) before coming to rest on a C major chord (indicated in orange). In between the blue and the orange, Williams inserts a few chords (E-flat and G) before resolving to the C major chord that concludes the introduction. These chords are entirely absent from the Mendelssohn. (As a side note, E-flat to G to C is exactly the same progression that George Harrison used in the introduction to "Something" on the Beatles' last-recorded album, Abbey Road.)
The Music of Star Wars
These posts will help focus and develop my analyses of John Williams' film scores.