Operation Flashpoint: Red River

I personally achieved this during Mission 7.

Shortly after using the humvee to meet up with the other Marines you will need to defend them while they fallback across the bridge. Once done you will be ordered to fallback across the bridge as well.

Shortly after you cross the bridge you will be tasked with defending the outskirts of a village from a big PLA assault. Order you team into a defensive position and hold the area. Once this task is done the achievement should pop as long as all prior objectives have been completed as well.

Me and my friend both got this completed after mission 7. We got the gold medal for completing all of the objectives and doing it in a certain amount of time so I think you have to just follow Knox's orders to the letter in mission 7 to get this achievement.

Achievement name: Rule #9
The description once achieving this says Orders

I obtained this randomly during mission 7 'Land in the Sand' Simply by following Knox's orders to the Letter.

This is one of the Ten Rules which all have achievements

Credit to Epsilon Theta
This is earned during mission seven after holding out at the crossroads after crossing the bridge.