What could IR feminist theory add to the understanding of war? Think, for example, about the ongoing US war in Afghanistan, from the point of view of IR feminism – does this war look any different? Does it change assessment/goals/constituents? (You can use any other war in this thought experiment)

One way of thinking about what IR feminist theory is would be to look at the field of International Relations through the lens of feminist theory. This means that feminist commitments to asking questions about gender and sexuality in particular—but of course always race, class, citizenship, ability, and many other identities—reshape the questions we ask in International Relations.
Here, we are talking about an IR perspective on war, which means we are interested in the ways in which states (i.e., nations) interact with one another in times of war. Feminist theory asks us to think about how these international relationships are informed by power, and how, in turn, that power is shaped by our perceptions and constructions of difference (e.g., categorizing people by gender, race, class, and nation).
For example, one justification given by the Bush administration for the US invasion of Afghanistan was that it was America's responsibility to save Afghani women from Afghani men. The thought behind this was that Afghani women, many of who wear the burqa (a form of Muslim veiling), were oppressed by an overt and violent patriarchal society (Jabbra). Indeed, it is often the case that pictures of suffering women and children are what circulate in the media in times of war, evoking feelings of compassion and humanitarianism within the public. One recent example of this can be seen in the way that pictures of women and children affected by chemical attacks in Syria have made their way to the tops of our news feeds. Another example is that during the 2015 European refugee "crisis," it was a picture of a little Syrian boy's body washed up on a Turkish shore that sparked outrage in the international community. (See, for example, the NPR report called "That little Syrian boy.")
A feminist reading of this particular justification would require asking one or more of the following questions:
Do Afghani women really need saving? Who is saying they need saving, and why?
What do Afghani women themselves actually think about "being saved"? Do they want to or need to be saved?
While the US government justifies their actions in Afghanistan by saying that a major reason for this war is to save Afghani women, what is happening that may contradict and/or challenge this so-called "justification"?
What binaries are being set up that might be based on false claims? For example, is the idea that all Afghani women are victims and all Afghani men are perpetrators warrant further thinking? What about the idea that the US is "more civilized" in that it is "more egalitarian" than Afghanistan? On what sort of preconceived notions about the relationship between Afghanistan and the US does this idea rely?
For further reading, I definitely recommend Cynthia Enloe's book Nimo's War, Emma's War: Making Feminist Sense of the Iraqi War. Enloe was one of the first feminist international relations scholars, and this particular book is all about women's perspectives and experiences of the Iraqi War.