  Numerous popular abstract strategy games ranging from Hex and Havannah to
Lines of Action belong to the class of connection games. Still, very few
complexity results on such games have been obtained since Hex was proved
PSPACE-complete in the early eighties. We study the complexity of two
connection games among the most widely played. Namely, we prove that Havannah
and TwixT are PSPACE-complete. The proof for Havannah involves a reduction from
Generalized Geography and is based solely on ring-threats to represent the
input graph. On the other hand, the reduction for TwixT builds up on previous
work as it is a straightforward encoding of Hex.
