A military strategy
in which force A surrounds all of force B, preventing its retreat
and cutting off its supply lines
. Force B's only real hope is in an attack. If the war is in a defensive paradigm
, then A has pretty much won. B still has a chance if it has allies
not enveloped, or if A is spread thinly enough that B can break out.
Hannibal used envelopment to defeat the Romans in the Battle of Cannae. In imitation of that battle, the German army tried to use envelopment in WWI, only on a more massive scale. Also, the Battle of the Bulge in WWII included an incidental envelopment of allied forces which Axis command had deemed irrelevant. Their holding on provided a propaganda boost to the allies... but that is another story.