Rape happens in wartime. It is a sad, ugly truth, and it is committed by all sides, even the ones that history remembers as righteous. But that being said, while it is somewhat uncomfortable as it can come off as equivocating about such a heinous act, nevertheless is must be stressed that there are differences in some of those rapes versus others, and it is important to discuss those differences to understand the difference between "mere" sexual violence in war time, and rape as a 'weapon of war' that you bring up, which is used to talk about the act as not merely being something that some soldiers did, and ideally would be punished for as a violation of their army's code, but instead as something that was expected to happen to some degree or other, whether explicit or implicit, and certainly little expectation of punishment or even attempts to curtail.

At this point I would note that while the title of the question is fairly strong warning already, this post will be talking a LOT about sexual violence. Please consider that before you choose to read. Likewise, those discussions at times in terms which might come off as detached, which is not intended to diminish their horror or leave out the voice of the victims, but does make it easier for the writer to get through a very depressing topic.

So you can pick essentially any war and likely find examples we could use, but I'm going to stick to World War II both due to a relatively high level of sources on the topic, and also my own familiarity. I'll start off with a brief discussion of the United States as it sets some useful context. As I said at the start, even the righteous side in a war - and I think there are few cases out there where that dichotomy is less controversial than with the Western Allies - have soldiers who commit it, and the US was no different. In point of fact, it must be stressed that the United States Army was not very good in how it handled rape. Certainly, it treated it as a crime, and it investigated accusations, brought soldiers to trial - at a higher rate than any other power in the war - and even executed them, but the process thoroughly racialized, as dealt with in more depth here.

Whatever the faults though, there was no policy of rape in the US military. Soldiers were not told to do it through official channels, nor tacitly encouraged to do so even if the official line said not to. Those soldiers who committed the act might have felt it unlikely that they would face consequences - a sad truth even outside of wartime - but they would not have in any way been able to say they were carrying out some sort of policy. Nor is this only restricted to the areas in which the US was liberator, but also true once they began entering Axis territory, where such a policy simply did not exist, and you simply won't find American propaganda or publications that encouraged it to be done to the enemy, however implicitly.

This gets to the heart of what 'weapon of war' means here, the difference between soldiers in an army doing terrible things because they are terrible people, and soldiers in an army doing terrible things because the army wants to terrorize the population and because it encourages them to do so one some level.

So from here we switch from the Americans to the Soviets. The mass rape of German women by the Red Army in 1944-45 is fairly well known, and something I've written about here. The numbers can be hard to know with precision, but there is no dispute that it was extensive, and certainly dwarfed the amount of sexual violence perpetrated by the Western Allies. But the numbers themselves are secondary for our purposes than is why and how it happened. Not that it excuses them, but certainly is important to keep in mind that the previous few years had been a war of annihilation, and the Nazi war machine had absolutely devastated much of the western Soviet Union under its occupation. This includes rape and sexual violence on an absolutely massive scale (and in ways which could also be used to as an example here, but I know the Soviet literature better). The Red Army, was absolutely vengeance minded as they entered Axis territory, and there was absolutely a driving desire to return the favor.

That is not to say that Stalin issues an order which stated "Rape their women like they raped ours" or something so explicit - something which apologists are quick to point out, but certainly some soldiers believed such an order existed. And in private Stalin absolutely was fine with such actions, remarking that it was fine "[i]f a soldier who has crossed thousands of kilometres through blood and fire and death has fun with a woman or takes some trifle". But even if that wasn't being printed on postcards for the soldiers, in any case it is not hard to find more tacit encouragement such as a poster that Merridale highlights, proclaiming "You are now on German soil. The hour of revenge has struck!" Merridale has a great passage I will quote as I think it well encapsulates just what is meant when someone says rape as a 'weapon of war':

Rape, then, combined the desire to avenge with the impulse to destroy, to smash German luxuries and waste the Fascists' wealth. It punished women and it reinforced the fragile manliness of the perpetrators. It also underscored the emotional ties between gangs of the men, and it was as a gang, not individuals, that the men usually acted, drawing an energy and anonymity from the momentum of the group. It was the collective triumph of these males, certainly, that rape purported to celebrate. And though women bore the brunt of the violence, German men were also victims of a kind. It was no accident that many rapes took place in view of husbands and fathers. The point was being made that they were now the creatures without power, that they would have to watch, to suffer this most intimate degradation

There is a ton going on in that passage, but the two threads which are most critical here is how rape occurred as a collective act, and how it was an act interwoven into the larger objectives of the Red Army entering Germany seeking punishment and retribution, and Soviet soldiers were often conscious of their brutality in this sense, such as one who wrote home that "Our soldiers have not dealt with East Prussia any worse than the Germans did with Smolensk", or a female soldier who was quite approving of her male compatriots behavior in noting "Our soldiers' behaviour towards Germans, particularly German women, is absolutely correct!'". Of course it also ought to be noted that Soviet policy did shift, although driven more by pragmatic concern than humanitarian one. Germany, after all, was not going to be literally obliterated from the map and the Soviets would eventually need to turn their eye to occupation, and building up the defeated power into a good socialist vassal state.

As such a shift can be seen in the Spring of 1945, with some commanders finally beginning to crack down on the sexual violence being perpetrated by their men, but serious punishment was quite rare - usually the actual charge would be for indiscipline, with rape almost entirely absent from Soviet military records - and often whatever punishment might be handed out (officially five year sentence) would be deferred and the soldier allowed to 'redeem himself' on the battlefield. As such, it can be seen that even when looked at as a problem, it was treated in a minor way.

The shift in the spring of 1945 also helps further illustrate the tacit approval that existed prior, illustrating an awareness of just what the earlier rhetoric had been encouraging. Ilya Ehrenburg is often the 'go to' for this, one of the most popular writers and propagandists of the war period whose sentiments would be well summed up as "The hour of revenge has struck!" (it ought to be stressed that while Ehrenburg's words absolutely can be seen as carrying an implicit message, the ascription of the explicit description of German woman as "lawful booty" was a creation of Nazi propaganda) and which were consumed en masse by the Red Army, but was expectedly told to walk much of it back in the spring, to be replaced by Stalin's sentiment that "the Soviet soldier will not molest a German woman. It is not for booty, not for loot, not for women that he has come to Germany" But the irony is that in so forcefully proclaiming such a bald faced lie, it only serves to be a stark contrast to the rhetoric of revenge and retribution that had feen fed unendingly to the Soviet soldiery prior. It wouldn't be until 1949 that serious prosecution and lengthy sentences for the soldiers began to occur in Soviet-occupied Germany.

So in short, that hopefully illustrates somewhat of the difference between rape in war, and rape as a weapon of war. All sides committed it, but only with some participants do we see policies intended to overlook, or even encourage it. While members of the US Army committed rapes during the conflict, in the end we can understand those as crimes of individual soldiers, not a crime of the Army. Meanwhile the members of the Red Army who engaged in such acts are best understood as part of a larger policy of brutality, which encouraged their behavior, and assured them that there would be little consequence for their behavior. The precise driving factors will vary from conflict to conflict, but in general such a framing means the acts of rape and sexual violence are best understood as something which was intended as one of the (likely) many terrors being visited upon the civilian population.