'To open the trenches' definitions:

Definition of 'To open the trenches'

From: GCIDE

Trench \Trench\, n. [OE. trenche, F. tranch['e]e. See Trench, v. t.]
1. A long, narrow cut in the earth; a ditch; as, a trench for draining land. --Mortimer. [1913 Webster]
2. An alley; a narrow path or walk cut through woods, shrubbery, or the like. [Obs.] [1913 Webster]
In a trench, forth in the park, goeth she. --Chaucer. [1913 Webster]
3. (Fort.) An excavation made during a siege, for the purpose of covering the troops as they advance toward the besieged place. The term includes the parallels and the approaches. [1913 Webster]
To open the trenches (Mil.), to begin to dig or to form the lines of approach.
Trench cavalier (Fort.), an elevation constructed (by a besieger) of gabions, fascines, earth, and the like, about half way up the glacis, in order to discover and enfilade the covered way.
Trench plow, or Trench plough, a kind of plow for opening land to a greater depth than that of common furrows. [1913 Webster]