I see some of the reasoning you have presented as a false dichotomy between the teaching of Christ, and the teaching of the Old Testament (although granted that there are differences between the situations). God commanded countless wars and killings in the Old Covenant, and the death penalty goes back (explicitly) at least as far back as Noah. As one who believes in the continuity of the Covenant of Grace, I see no dichotomy between loving your enemies, and a time for war/defense of one's self. Part of the sixth commandment includes the duty of
also the duty of
(WLC Q135) all careful studies, and lawful endeavors, to preserve the life of ourselves  and others ...by just defense thereof against violence,
1. Eph. 5:28-29
2. I Kings 18:4
8. Psa. 82:4; Prov. 24:11-12; I Sam. 14:45
(WLC Q136) lawful war, or necessary defense;Anything else is breaking the sixth commandment and should be avoided. We should love our neighbors as ourselves, but is it loving ourselves/countrymen if we do not take up arms to defend ourselves? We should love our enemies to be sure, but this does not mean that we should become door mats.
4. Jer. 48:10; Deut. ch. 20
5. Exod. 22:2-3
Also, the church is not one that should wage war against enemies in just situations. That is the job/role of the civil magistrate.