  We consider environment induced decoherence of quantum superpositions to
mixtures in the limit in which that process is much faster than any competing
one generated by the Hamiltonian $H_{\rm sys}$ of the isolated system. This
interaction dominated decoherence limit has previously not found much attention
even though it is of importance for the emergence of classical behavior in the
macroworld, since it will always be the relevant regime for large enough
separations between superposed wave packets. The usual golden-rule treatment
then does not apply but we can employ a short-time expansion for the free
motion while keeping the interaction $H_{\rm int}$ in full. We thus reveal
decoherence as a universal short-time phenomenon largely independent of the
character of the system as well as the bath. Simple analytical expressions for
the decoherence time scales are obtained in the limit in which decoherence is
even faster than any timescale emerging from the reservoir Hamiltonian $H_{\rm
res}$.
