Question: is as soon as a preposition?
Answer: Prepositions: (1) after, when, while, before, as soon as, as long as, once, at, between, by, ago, into, on, once, for, since, during, in, toward(s), until, and (2) w/o object complements now, then and when (3) fused with a noun: afterward(s), beforehand, henceforth, awhile.