described in a series of short stories and novellas by Fritz Leiber
. Two shadowy alien
races, commonly referred to as the Spiders
and the Snakes
, are embroiled in a war
for supremacy in which the battleground is history
itself. They recruit humans as their agents
, cutting them loose from their lifelines the moment before their death
, and send them through Time
to change history in a way that gives their side the advantage.It's a tough job since history is resistant to change - if, for example, you were to kill Hitler someone exactly like him would simply take his place and do exactly the same things.
The stories rarely focus on the battles themselves, and are instead usually set in the Places, rest and recuperation centers for wounded and battle-fatigued soldiers outside the timestream.