3.cxd5 Nxd5 4.e4
A common continuation, though White may be playing e4 too early. If Black deviates with 3...Qxd5, 4.Nc3 Qa5 5.Bd2 is strong, e.g. 5...Qb6 6.Nf3 Qxb2?? 7.Rb1 Qa3 8.Nb5, winning (Alburt 2009:38).
After Black retreats the knight with 4...Nf6, White can continue 5.e5 attacking the knight, or he can get a clear advantage with 5.f3, or a small advantage with 5.Nc3 e5! 6.Nf3! (6.dxe5 Qxd1+ 7.Kxd1 Ng4!) 6...exd4! 7.Qxd4 (Alburt 2009:38).
Read more about this topic: Marshall Defense