Another way that the pacifist reading of the Sermon on the Mount has been countered is to say that Christ was not referring to enemies of the public, but rather private enemies.
But: just because you become the enemy of a criminal because you are a magistrate, does not make them any less your enemy. That is, the private/public distinction does not really work in this case, as magistrates are doing harm to people (by the nature of the case, or at least by convention), and that makes those people de facto their enemies. The reason they are their enemies, that they are representatives of the public and thus take upon themselves the behaviour of avenging the public (as opposed to, say, being enemies because someone directly attacked the individual who is the magistrate), does not change the fact that they are their enemies.
Thus, one can’t say that magistrates avoid breaking Christ commands because they are not doing harm to their enemies, but someone else’s, because they have made someone else’s enemy their enemy.
This means that, basically, one can only avoid the pacifist reading by saying that Christ implicitly meant that magistrates were allowed to hate some of their enemies, while everyone else could not hate any of their enemies. As this significant qualification is not on the face of the text of Matthew, it must come from the context, i.e., in necessary truths, in the canon (for those for whom the Canon is authoritative), or in the historical context of Christ’s words. I doubt that the first context can provide that qualification, so that leaves the latter two: either this qualification must be justified on the basis of the canon, or because of the historical context of Christ’s words.
So here’s the challenge for non-pacifists: either show that the canon says that magistrates will, for all time (prior to the parousia, presumably), have to hate some of their enemies (so that Christ’s command could not possibly rule that out without contradicting this), or else show that some part of Jesus’ historical-linguistic context would have implied that when Jesus said “love your enemies”, he meant “love your enemies, unless you are a magistrate, then love your enemies in some situations and hate them in others.”