  We introduce the strong alliance polynomial of a graph. The strong alliance
polynomial of a graph $G$ with order n and strong defensive alliance number
$a(G)$ is the polynomial $a(G;x):=\sum_{i=a(G)}^{n}\, a_i(G)\ x^i$, where
$a_{k}(G)$ is the number of strong defensive alliances with cardinality $k$ in
$G$. We obtain some properties of $a(G; x)$ and its coefficients. In
particular, we compute strong alliance polynomial for path, cycle, complete,
start, complete bipartite and double star graphs; some of them verify
unimodality.
