Infiltration is the most critical addition to LW2. When you launch a mission, your squad "waits" at the site to prepare.
Aim for at least 100% infiltration on every mission. Launching at 0% will result in "Swarming" enemy activity, making victory nearly impossible. xcom 2 long war guide
LW2 combat is a game of "Alpha Striking"—killing or controlling the enemy before they can act. Infiltration is the most critical addition to LW2