  We prove that when subjected to periodic forcing of the form $p_{\mu, \rh,
\om} (t) = \mu (\rh h(x,y) + \sin (\om t))$, certain second order systems of
differential equations with dissipative homoclinic loops admit strange
attractors with SRB measures for a set of forcing parameters $(\mu, \rh, \om)$
of positive measure. Our proof applies the recent theory of rank one maps,
developed by Wang and Young based on the analysis of strongly dissipative
H\'enon maps by Benedicks and Carleson.
