They set off, two men out in front as scouts, two behind in case of any attack from the rear.
他们出发了，两人在前作为侦察员，两人殿后以防后方攻击。
《柯林斯英汉双解大词典》
2
If you know someone is coming up behind you to attack, turn toward the person with your hands up in front of your body and loudly say "stop" before walking away.
如果你知道有人从你后面走过来要攻击你，把手举到身前，转身大声说“停”，然后走开。
3
As Israel conducts a big civil-defense drill this week, Syria will hold an unprecedented exercise to test the readiness of the home front for a possible Israeli attack.