MathOverflow will be down for maintenance for approximately 3 hours, starting Monday evening (06/24/2013) at approximately 9:00 PM Eastern time (UTC-4).
show/hide this revision's text 1 [made Community Wiki]

Quadratic reciprocity feels to me like a "first nontrivial statement" without an obvious branch of mathematics. Not number theory -- there are things like the infinitude of the primes, and "algebraic number theory" doesn't seem quite right either...