The Scrolls of Lankhmar: TheChangeWar

The Change War stories consist of the short novel The Big Time and several short stories:

Try and Change the Past (in the anthology Changewar)
The Oldest Soldier (in the anthology Changewar)
Damnation Morning (in the anthology Changewar)
When the Change-Winds Blow (in the anthology Changewar)
Knight to Move (in the anthology Changewar)
A Deskful of Girls (in the anthology Changewar)
No Great Magic (in the anthology Changewar)
The Number of the Beast
Black Corridor

They concern the spacetime-spanning war between two factions, the Spiders and the Snakes, who alter and re-alter history in their battles. The war is so vast, the human combatants, and the non-human ones encountered by the humans, do not even know what the issues of the war are, or what their complete chains of command are.
One interesting feature of the Change War setting is the Big Time. This is a second dimension of time, the one in which history changes. (Cf. "Eternity" in Isaac Asimov's time-travel novel, The End of Eternity.) The Big Time lets Leiber avoid the logical snarls that result in most time-travel stories that permit changeable history.
People are recruited into the Change War just before their deaths. If history changes to move your time of death earlier, you wink out. If your time of death becomes later, you may be left in the uncomfortable position of remembering your future. This on top of all the more usual ways to die.

The Scrolls of Lankhmar : TheChangeWar