A “preventive war” is a war undertaken proactively against a merely potential enemy, who has neither initiated hostilities nor shown any sign of intending imminently to do so. The Japanese attack on the United States at Pearl Harbor is a famous example. This is not to be confused with a “preemptive war,” which involves a proactive attack on an enemy who has shown signs of intending to initiate hostilities. The Arab-Israeli Six-Day War is a standard example.
The Iraq war of 2003-2011 was sometimes characterized as a “preventive war,” though in my opinion that is, whatever else one thinks of that war, not an accurate characterization. Rather, I think it fell under the category of “punitive war,” a war fought to punish an enemy nation for some offense (such as a violation of treaty obligations). Whether it was justifiable under that description is not an issue I am addressing here. What is relevant is that critics of the Iraq war who characterized it as a preventive war took it to be ipso facto unjust. For while preemptive war is generally thought to be justifiable, preventive war is – rightly, in my view – widely thought not to be justifiable.
The reason should be obvious. Until a potential enemy has actually done something – such as actually attacking (which would justify a defensive war), or preparing to attack (which could justify a preemptive war), or in some other way actually committing a sufficiently grave offense (which might justify a punitive war) – said potential enemy is in all relevant respects innocent. You cannot justifiably attack a nation merely for what it might do, any more than you can punish an individual for what he might do.
This is why we don’t arrest and punish gangsters even when we have good reason to suspect that they will at some point commit crimes, and don’t fine corporations even when we have good reason to suspect that they will at some point pollute. You can justifiably inflict harm on people only for what they have in fact done, not for what you think they probably will do in the future, and certainly not for what they merely might do.
But don’t we rightly punish people for certain negligent acts, even when they don’t actually result in harm? Yes, but that is because such punishments are relevantly analogous to preemptive war rather than to preventive war. Suppose I use a flamethrower to clear away brush or scare off raccoons in my backyard. Suppose I don’t actually end up igniting your yard or house. I still have in fact put your property in imminent danger of harm, even if I had no hostile motive but was just being stupid. And it is reasonable to forestall actions that are per se dangerous in this way by prohibiting them altogether, as well as by punishing them after they occur.
It would not be reasonable, though, to prohibit ownership of (say) chainsaws, merely because someone might be so stupid as to use them in a way that endangered others. It is very difficult to use a flamethrower in your backyard in a way that does not pose an imminent grave risk to your neighbors. But it is not difficult to use a chainsaw in a way that poses no serious risk to others. Sure, I could do something really stupid with it – say, tying it to a rope, starting it up, and then swinging it around in a wide arc that crosses over your property line – but it is extremely unlikely that many if any chainsaw owners would do such a thing. Flamethrower use in a neighborhood context is per se dangerous to others in a way that chainsaw use is not.
Now, this is the principle on which quarantining disease carriers is justifiable, at least when walking around with the disease is more like using a flamethrower than it is like using a chainsaw. Hence, it is reasonable to quarantine people with bubonic plague. But it would be unreasonable to quarantine people with the flu, even if occasionally there are people who die from the flu. Quarantining someone with bubonic plague inflicts a harm on him – it takes away his freedom of movement and may thereby prevent him from making a living or going to school, cause emotional distress, and so on – but this is justifiable given that his walking about freely would impose a grave and immediate threat to others, just as using a flamethrower in your backyard would. Quarantining such a person would be analogous to a preemptive war – the forestalling of a grave and imminent threat that the person actually does in fact pose.
But it would not be reasonable to quarantine a person simply because he might get bubonic plague and pass it to others, or because he does in fact have an illness but one which merely might cause grave harm to another (such as the flu or a severe cold). That would be analogous to a preventive war rather than a preemptive war, and illegitimate for the same reason. You can justifiably quarantine Typhoid Mary. But how can you justifiably quarantine Potentially Typhoid Mary, any more than you can justifiably attack a potential enemy? Or how could you justifiably quarantine Severe Cold Mary on the grounds that some people might in theory die if they catch her cold, any more than you could legitimately ban chainsaws on the grounds that someone somewhere might use a chainsaw foolishly?
Now, COVID-19 is not remotely like bubonic plague, and while for some people it is certainly worse than the flu, for most people it is not. And we know who is most vulnerable – the elderly and those with certain preexisting medical conditions. So, how can it possibly be justifiable to quarantine those who do not have the virus, on the grounds that they might get it, and then might go on to spread it to someone among the minority of people to whom it poses a grave danger? Especially when there is an obvious far less draconian alternative, namely quarantining only those who do have the virus and those who are at special risk from it? And especially when there is no proof that the more draconian measures are really necessary, and evidence that in fact they have
In short, how are lockdowns for vast populations of healthy people any more justifiable than “preventive war”? How is the argument “If we don’t quarantine the healthy, grandma might die if they catch the virus and spread it to her” any better than the argument “If we don’t proactively attack country X, grandma might die if X attacks us”? If those who start a “preventive war” are war criminals, what are those who have “locked down” the healthy and thereby destroyed livelihoods, inflicted severe mental distress, and set back the education of millions of children – and all for nothing, given the evidence that such policies have at the end of the day
Don’t answer: “But killing people in a war is worse than quarantining them!” Of course it is, but that’s irrelevant. Destroying the livelihoods, etc. of innocent people is not as bad as killing them, but it hardly follows that it isn’t extremely bad. And since when is a government morally permitted to inflict whatever damage it sees fit on innocent citizens, as long as it stops short of killing them?