Valhalla (Stargate SG-1, #14)

SG-1 become enmeshed in a fierce battle straight out of Nordic mythology. Although unwilling participants, SG-1 soon find themselves fighting with the Vanir against their ancient foe - the brutal giants of Jotunheim - and feasting with them long into the night. But when dawn arrives, the team find themselves in a very different world. Although still at war, the Vanir and Jotunheim have advanced a millennia and fight with weapons of the 18th century.