  Let $\Gamma$ be a compact tropical curve (or metric graph) of genus $g$.
Using the theory of tropical theta functions, Mikhalkin and Zharkov proved that
there is a canonical effective representative (called a break divisor) for each
linear equivalence class of divisors of degree $g$ on $\Gamma$. We present a
new combinatorial proof of the fact that there is a unique break divisor in
each equivalence class, establishing in the process an "integral" version of
this result which is of independent interest. As an application, we provide a
"geometric proof" of (a dual version of) Kirchhoff's celebrated Matrix-Tree
Theorem. Indeed, we show that each weighted graph model $G$ for $\Gamma$ gives
rise to a canonical polyhedral decomposition of the $g$-dimensional real torus
${\rm Pic}^g(\Gamma)$ into parallelotopes $C_T$, one for each spanning tree $T$
of $G$, and the dual Kirchhoff theorem becomes the statement that the volume of
${\rm Pic}^g(\Gamma)$ is the sum of the volumes of the cells in the
decomposition.
