  The dynamics of a disordered nonlinear chain can be either regular or chaotic
with a certain probability. The chaotic behavior is often associated with the
destruction of Anderson localization by the nonlinearity. In the presentwork it
is argued that at weak nonlinearity chaos is nucleated locally on rare resonant
segments of the chain. Based on this picture, the probability of chaos is
evaluated analytically. The same probability is also evaluated by direct
numerical sampling of disorder realizations and quantitative agreement between
the two results is found.
