  We study the ground state properties of the bond alternating $S=1/2$ quantum
spin chain whose Hamiltonian is H=\sum_j (S_{2j}^x S_{2j+1}^x +S_{2j}^y
S_{2j+1}^y +\lambda S_{2j}^z S_{2j+1}^z ) +\beta \sum_j {\bf S}_{2j-1} \cdot
{\bf S}_{2j} . When $\beta=0$, the ground state is a collection of local
singlets with a finite excitation gap. In the limit of strong ferromagnetic
coupling $\beta \to - \infty$, this is equivalent to the $S=1 \ XXZ$
Hamiltonian. It has several ground state phases in the $\lambda$-$\beta$ plane
including the gapful Haldane phase. They are characterized by a full breakdown,
partial breakdowns and a non-breakdown of the hidden discrete $Z_2 \times Z_2$
symmetry. The ground state phase diagram is obtained by series expansions.
