it is "lying in wait" not "lie in wait". It means that the enemy (soldiers) are hiding along the route ready to attack or surprise them. AMBUSH NOUN a surprise attack by people lying in wait in a concealed position. to use "lie in wait" you have to change the tense. "Don't send your soldiers on that route. I heard your enemies will lie in wait to ambush them along the way." "lying in wait" = they are already hiding ready to ambush or known to be already hiding. "will lie in wait" = it is known/believed/suspected/ pre-supposed/guessed that the enemy will do this or are very likely to do this.

'lie in wait to ambush' means to hide quietly behind cover where you can't be seen, so that you can surprise and start attacking someone without them having warning.