Let E/Q be an elliptic curve, let p > 2 be a prime of good reduction for E, and assume that E admits a rational p-isogeny with kernel Fp(Ļ). In this paper we prove the cyclotomic Iwasawa main conjecture for E, as formulated by Mazur [Maz72], when Ļ| Gp = 1, Ļ, where Gp is a decomposition group at p and Ļ is the TeichmĆ¼ller character. Our proof is based on a study of the anticyclotomic Iwasawa theory of E over an imaginary quadratic field K in which p splits, and a congruence argument exploiting the cyclotomic Euler system of Beilinson-Flach classes.