How about have a map in which when time is turn "n", when the empire start
an attack on somewhere more important than which the player is playing,
and the player is assigned to deal with a bigger battle of another map?
This way, one may set the victory condition as "turn=20" then a message like
"The empire is on their way to our HQ !" , then the game of this map is over,
and the player is sent to play another map, after victory of that map, or certain
turns, say turn=15 (of course, these maps should be big enought not to let the player
win too fast) the player is informed that the battle in last map is lost.
Map
A Turn=20 or so -> player is sent to map B
B Turn=15 or so -> player is informed the battle in map A is lost