  We describe a weak coupling realization of the deconfinement transition in
gauge theory compactified on $R^3\times S^1$. We consider Yang-Mills theory
with a single Weyl fermion of mass $m$ in the adjoint representation of the
gauge group. The fermion is subject to periodic boundary conditions,
$\lambda(0)=\lambda(L)$, where $L$ is the size of the circle $S_1$. This theory
reduces to thermal Yang-Mills theory in the limit $m\to\infty$. In the limit
$m\to 0$ the deconfinement transition can be studied using weak coupling
methods. The analysis is based on semi-classical objects characterized by
topological and magnetic charges. At leading order the relevant configurations
are monopole-instantons and monopole-anti-monopole pairs ("bions"). We argue
that in the $m-L$ plane the weak coupling transition is continuously connected
to the deconfinement transition in pure gauge theory.
