We do not know that Paul carried no sword, however against an angry mob, a sword wouldn’t be much help. In any case, whether or not Paul carried a sword, Jesus saw fit to advise His disciples to purchase one even if it meant selling that one’s cloak. The sword goes along with the purse and etc. The disciples didn’t require them on their earlier journeys but they WILL need them on subsequent occasions.
Regarding Jesus saying “I didn’t come to bring peace, but a sword,” this was a metaphor meant to warn His listeners that He would be a cause of contention and division even in the closest of relationships.
Regarding nations going to war, the idea that the USA is not “in Christ” is irrelevant. The man or woman who gives the order to launch a counter strike is a human being and either in Christ or not yet in Christ. If he is in Christ, is he thereby forbidden to come to the defense of his countrymen? Is it then immoral for a president who is a Christ follower to give the order to defend the nation? Or is it okay to give the order but not to personally pull the trigger? By this logic, if we’re to be consistent, all LEOs must be non-Christians and Christians must not ask to be defended by them, because that would be no better than sin by proxy. If the POTUS (or similar) is a Christian, wouldn’t it be hypocritical for her/him to accede to being accompanied by armed bodyguards? You see the logical extremes to which this extra-biblical doctrine takes us?
The sword is for self defense. It’s the only logical conclusion. If it offends your personal ethic, then don’t carry a “sword.” As for myself I’d never carry a sword except maybe as a costume accessory. I prefer a Shield. It’s much easier to conceal.