I fought for the US in Iraq and in Afghanistan. I can’t share much insight into the CCC, but I might be able to clarify some of the principles from a combatant’s point of view. There are three primary sets of documents which govern the legality and the propriety of acts of war for US military personnel. The UCMJ (Uniform Code of Military Justice), The Geneva Conventions of 1949 and their subsequent amendments, and the ROE (Rules of Engagement).
The UCMJ is the legal code for the US military, which governs conduct ranging from behavior appropriate to the uniform, to personal moral conduct which the military feels should fall under its purview, to professional conduct, including conduct during war and combat.
ROE are specific for any war or engagement, and are devised and promulgated by the military leadership in consultation with the department of defense and the president or his representatives. ROE are learned intimately and completely by each combatant before engaging the enemy.
One thing to bear in mind, with respect to any of these sets of rules, is that the state of mind of the combatant is considered foremost when the rule of law or principles of conduct are applied. The question is not posed as “what was the best act in retrospect?”, The question is: “what was the correct act under the circumstances and given the state of mind of the person acting.”
I will give a couple of examples of how the civilian view of these governing rules and principles might be different for militants than for civilians.
In this video:
youtube.com/watch?v=3KnH2t3Gjio&NR=1 you can see a man lying on the ground playing dead. You can also see my back at one point

The Marine in the clip who had assumed that he was dead, notices him start to move. Calls out the warning, “He is playing dead,” and shoots him. To the noncombatant this may appear to the unjustified killing of an innocent man. However, “playing dead” is prohibited under the Geneva Conventions. The Marine’s actions are justified in my opinion, as the man posed a very clear threat. I leave it to others to decide how much risk a combatant should take in such a situation.
Here is a second example from OIF1. A unit of about 1,000 men had many of their number pinned down under fire. An RCT (Regimental Combat Team) comprising about 6,000 men with armor and artillery were called up for reinforcement. My unit, a 550 man reconnaissance unit was called up for assistance in extraction of personnel pinned down under fire. My unit arrived prior to the slower moving RCT, and immediately came under mortar attack. Scouts were sent forward to eliminate the threat, but in the meantime we had to fall back or endure the attack. Mortars are popular because they are very portable, so can be fired from civilian roof tops, etc… and they have long range… more than a mile. The disadvantage is that the shooter sometimes cannot see his target. In that case, the shooter will be teamed with a spotter, who has eyes on the target. The spotter communicates back to the shooter to change his targeting in a feedback loop as he observes the target.
As we had mortar shells landing and drawing closer, we also had identified the spotter. He was watching us, and popping in and out of a doorway, talking on a phone. As each salvo was coming in, he would be watching us from his doorway and talking on the phone. The fire was getting more accurate. We were not allowed to engage the spotter under our ROE which prohibited targeting unarmed civilians. The ROE were specific that all targets must have a weapon in their possession. So, we requested hot authorization to change the ROE for this circumstance, and waited for a reply. Our unit commander, who was a full colonel, did not have sufficient authority to change the ROE.
When the authorization came down, there was a large volley of fire, which not only killed the spotter, but also did significant damage to the poorly built building he was in. In the future, when we had a spotter to deal with, which was a regular occurrence, we would call a sharp shooter forward to handle it. Sometimes it was difficult to determine precisely if someone was a spotter or not. In one case, we were not sure, but a judgment was made based on available information, and he was ordered shot. The mortar fire stopped.
I leave it to others to decide where in the CCC these moral questions lie. Was it immoral to have destroyed a building under those circumstances? Was it immoral to kill a person for whom the weight of the evidence showed him to be a spotter? He might not have been. In this case he was.
Finally, I would like to point out that the US military engages enemies as ordered by the civilian government. It does not pick and choose which wars to fight. The duty of a military person is to follow these orders to the best of his ability. The bias is that the orders are moral and correct. There is a moral obligation which comes with taking life or destroying property, etc… The personal responsibility is spelled out in the UCMJ, GC, and ROE. If a US military person honestly objects on moral grounds to deploying for a particular engagement, he may conscientiously object, and his objection will be considered. If his objection is declined, then he may either choose to deploy, or he may choose to desert or disobey an order. The penalty can range from loss of rank and pay, to imprisonment, dishonorable discharge, or death. Desertion at time of war is a capital crime. Cowardice is a capital crime, under the UCMJ. Desertion is defined as being away without the intention of returning to duty. Intent is assumed after 30 days of absence, if not provable by other means. So, the decision not to fight, is a very serious one.
Perhaps an expert on the Catholic position would like to take up some of these issues