The beginning of the overture sounded familiar, but I had to look up the notation before I realized why it sounded familiar:
Initially, I thought it resembled the melody of Handel's "See the conqu'ring hero comes" from Judas Maccabaeus (I know it as a melody, but I've never heard Handel's oratorio). After I lookt up the notation for "See the conqu'ring hero comes," I realized that Mendelssohn's overture sounds familiar because it starts with the same rhythm, not the same melody:
Both have a half note, a dotted quarter note, an eighth note, and then two half notes.
After I discovered this similarity, I did some research to see if this was actually Handel's influence or whether it was just a coincidence. I referenced R. Larry Todd's Mendelssohn: A Life in Music, which I read last year. In the index, there are a dozen entries for "Handel influences." I lookt up Judas Maccabaeus specifically, and - while I couldn't find anything about its influence on the Athalia overture - I did find some things that evidence Mendelssohn's familiarity with it. In Chapter 5 (1825-1826), Todd writes that "Felix extracted themes from the oratorio and dexterously wove them into an improvisation, 'thoroughly Handelian' yet with 'no pretension to display.'" Later, Todd mentions Judas Maccabaeus among other Handel works that Mendelssohn conducted in Düsseldorf in the 1830s. So it seems that the rhythm from "See the conqu'ring hero comes" could very well have influenced the rhythm in the overture to the Athalia incidental music.
