One of these, first in chronological order, is for a fixed number of months to cause systematic losses in the enemy's offensive capacity. This tactic is carried out on the vanguards. Unfavorable ground impedes flank defenses by the advancing columns; therefore, there must always be one point of the vanguard that, as it penetrates and exposes the lives of its components, serves to give security to the rest of the column.
(Guerrilla Warfare; Ernesto "Che" Guevara)
Should I read it as follows:
"On unfavorable ground the advancing columns can't effectively protect the flanks."