However, a thorough consideration of the requirements of the sixth commandment not only fails to support pacifist claims against any possibility of just war, they actually oppose such claims in providing the basic justification for waging war.
In the Westminster Larger Catechism, question 135 supplies a clue:
Question 135: What are the duties required in the sixth commandment?
Answer: The duties required in the sixth commandment are, all careful studies, and lawful endeavors, to preserve the life of ourselves and others by resisting all thoughts and purposes, subduing all passions, and avoiding all occasions, temptations, and practices, which tend to the unjust taking away the life of any; by just defense thereof against violence, patient bearing of the hand of God, quietness of mind, cheerfulness of spirit; a sober use of meat, drink, physic, sleep, labor, and recreations; by charitable thoughts, love, compassion, meekness, gentleness, kindness; peaceable, mild and courteous speeches and behavior; forbearance, readiness to be reconciled, patient bearing and forgiving of injuries, and requiting good for evil; comforting and succoring the distressed, and protecting and defending the innocent.
Notice that the clause about resisting the unjust taking of life is joined to the clause about just defense against violence by the preposition, "by," which in this case most liking is talking about agency, that is, one resists those things that tend toward the unjust taking away of life by means of a just defense against violence.
It does not take long to think of scenarios in which the taking of another's life would be necessary in order to defend the unjust taking of life by that other. With war in particular, the application of the sixth commandment is the protecting of a political community against a hostile opposing political community through defensive response to aggression. Ideally, defensive response would not result in death, but in such cases where death does result from defensive opposition to the unjust taking away of life, then the culpability for that life falls upon the aggressor, not the defensive party.
Consider a very non-aggressive defensive example. Let us say that First City learns of a plan of attack against them by Second City. First City then builds a large wall of defense around their city and gathers supplies and provisions for an immanent siege. Second City marshals its troops and arms and approaches First City to conquer it, unprovoked. First City has no long-range weapons, so they do not attack Second City directly, but bolster their doors and place men with swords at the doors and on the walls. Second City has brought several war devices, including ladders and grappling hooks to scale the wall. In the process of setting up these ladders and hooks, several of Second City's troops do a poor job of setting their ladders and securing their hooks, and they plummet from the wall to their own deaths.
Would any pacifist consider the actions of building the wall around the city to be an unjust act of violence by the First City? Yet such an act is in fact an act of warfare, albeit defensive in nature, since it was in response to the aggressive intention of Second City. First City could have simply sat back and prayed for God's mercy, as opposed to building the wall to protect its citizens. But let us take the example one step further. Let's say that as the ladders and grappling hooks are being set, several of the men of First City unhook the hooks and unhinge the ladders, which leads to the deaths of several soldiers from Second City who plummet to their deaths. Again, the action is defensive--preventing an armed enemy from entering the city by removing his means of entry. The pacifist is probably less likely to agree with this action, the only difference is the amount of agency exerted by the defensive party in preventing the aggressor from harm. The defensive posture of the action is unchanged, as is the object of protection. The men of First City might even yell to their enemies prior to unhinging and unhooking in order to offer their enemies an opportunity to survive.
The main problem with pacifism is that it tends to consider only the life of the aggressor and ignores the life of the party or parties that face the violent threat of the aggressor. Even when the life is one's own, one is responsible to protect that life, for one's own life belongs to God, and not to oneself. To allow a violent aggressor to take one own's life without engaging in a "just defense thereof against violence," is so far from keeping the sixth commandment, but rather is a breaking of it!
Certainly the topic of just war is much more complicated that what I've expressed here, but I'm not attempting to consider the particulars of waging just war, so much as I am interested in showing the basic justice of war, as it is entailed in the sixth commandment. One cannot protect and preserve life without being willing to take away life when the life one must take threatens the life one is seeking to protect and preserve. Ultimately, we must remember that God calls us all to be as He is, a Defender of the weak and defenseless against the mighty who would oppress and kill them unjustly.