  It is shown that for any action of a finitely presented group $G$ on an
$\R$-tree, there is a decomposition of $G$ as the fundamental group of a graph
of groups related to this action. If the action of $G$ on $T$ is non-trivial,
i.e. there is no global fixed point, then $G$ has a non-trivial action on a
simplcial $\R $-tree.
