defensive perimeter; defense line
Literal military or physical defensive line. Also used figuratively for measures taken to prevent something.
They set up a defensive perimeter to prevent the enemy's invasion.
precaution; safeguard; safety measure
Almost always used in the phrase 予防線を張る, meaning to take precautions or lay the groundwork to avoid trouble or criticism.
See also: 予防線を張る
He took precautions in case he failed.
I lay a safeguard before my boss can object.