First I should be careful to note that not all reactionaries, or even necessarily a supermajority of such will agree with what I have to say.
War, especially modern war, is a horror. It is a terrible thing, although it is not necessarily the worst thing. Being conquered, for instance, is usually a worse thing. But I can't say I've lived through a war (albeit they're not usually named that), that from the perspective of a superpower like the US, was advantageous materially or morally to get involved in.
I subscribe, as I suspect do most reactionaries, to the all or nothing school of warfare. Either you're willing to commit the requisite blood, treasure, and bloodshed to accomplish your stated objectives in a conflict or you are not. If you are, and can reasonably foresee your will persisting over the course of the conflict, then perhaps you can justify engaging in said conflict. If not, don't even try. Don't just do something, stand there.
At the beginning of any conflict when you're contemplating getting involved, consider these things. How much is this, in dollars and lives on our side, going to cost? Then, how many of Them is it going to be necessary to kill or maim to compel capitulation to our aims? Then ask the question, is this calculus acceptable to us? Some case studies are useful in this regard---consider Germany and Japan. Both of them were opponents with strong morale and a profound dislike for being conquered. Both capitulated more or less unconditionally. Both required an immense amount of blood and treasure to defeat and suffered immense numbers of casualties themselves.
Having this debate in open and honest terms is likely to forestall much nonsense about rules of engagement should you actually decide that you have to fight. The US WWII and prior knew how to actually conduct a conflict of this type and more or less invariably succeeded in its aims. Moldbug, for instance, has linked to a number of WWII and prior documents produced by the US army as regards occupation and similar unpleasantness. But I think you'll find that most of the time, you'll decide that the best answer is not war.
War is also immensely socially destructive, especially when large numbers of casualties are involved. For one thing, it disrupts the natural male-female ratios profoundly, often resulting in the breakdown of moral standards. It typically also involves massive amounts of debt, which frequently spreads a large amount of moral hazard all throughout society like shrapnel as well. Frequently that debt ultimately destroys currencies. Lastly, it drags all the worst impulses of rent-seeking out of the population. Is it any wonder that a lot of the 2nd wave of entitlements came out of the Vietnam era? I think not. Lastly, there is this point: when you are a superpower, the threat of what you would be able to do, when it is not manifest obviously, is frequently far scarier than the reality. The British found this during their days of Empire---getting entangled in one war frequently led to becoming mired in several more because the awe of your incredible power was diminished in the eyes of the opportunistic. A sword is best used to keep another in its sheath.
Of course this last point is nearly unthinkable to Americans---sometimes, you may find that your aims are not achievable at the price that you're willing to pay. In layman's terms, you might lose. Losing is nearly universally agreed to suck, even when it fails to be accompanied by enemy soldiers goose-stepping down your avenues. One could compress nearly the entire message of 'The Art of War' from the perspective of a major power into this: Never get your country involved in a war that will later be of interest to wargamers.