  Let $p$ be a prime. Let $V$ be a discrete valuation ring of mixed
characteristic $(0,p)$ and index of ramification $e$. Let $f: G \rightarrow H$
be a homomorphism of finite flat commutative group schemes of $p$ power order
over $V$ whose generic fiber is an isomorphism. We provide a new proof of a
result of Bondarko and Liu that bounds the kernel and the cokernel of the
special fiber of $f$ in terms of $e$. For $e < p-1$ this reproves a result of
Raynaud. Our bounds are sharper that the ones of Liu, are almost as sharp as
the ones of Bondarko, and involve a very simple and short method. As an
application we obtain a new proof of an extension theorem for homomorphisms of
truncated Barsotti--Tate groups which strengthens Tate's extension theorem for
homomorphisms of $p$-divisible groups.
