The ego is the first line of defense between the self and the outside world.

VOA: special.2010.04.13

The Germans,by a remarkable combination of air bombing and heavily armored tanks, have broken through the French defenses north of the Maginot Line, and strong columns of their armored vehicles are ravaging the open country, which for the first day or two was without defenders.

VOA: special.2011.05.12

He says agents working at airports, seaports and border crossings are often the first line of defense for victims.

VOA: standard.2010.07.23

It also eases the burden on police and security forces as a first line of security.

VOA: standard.2009.08.31