Let E/Q be an elliptic curve, and p a prime where E has good reduction, and assume that E admits a rational p-isogeny. In this paper, we study the anticyclotomic Iwasawa theory of E over an imaginary quadratic field in which p splits, which we relate to the anticyclotomic Iwasawa theory of characters following the method of Greenberg-Vatsal. As a result of our study, we obtain a proof, under mild hypotheses, of Perrin-Riou's Heegner point main conjecture, as well as a p-converse to the theorem of Gross-Zagier and Kolyvagin and the p-part of the Birch-Swinnerton-Dyer formula in analytic rank 1 for Eisenstein primes p.