  By a theorem of Suslin, a Tor-unital (not necessarily unital) ring satisfies
excision in algebraic K-theory. We give a new and direct proof of Suslin's
result based on an exact sequence of categories of perfect modules. In fact, we
prove a more general descent result for a pullback square of ring spectra and
any localizing invariant. Besides Suslin's result, this also contains Nisnevich
descent of algebraic K-theory for affine schemes as a special case. Moreover,
the role of the Tor-unitality condition becomes very transparent.
