The Solferino fallacy is the assumption that war is something that just naturally happens, and laws are enacted from the outside to try to regulate it. It is a misunderstanding of what international humanitarian law, like the Geneva Conventions, is and how it operates. The fallacy lies in the proposition that war is natural. While conflict may be natural, "war" is a specific type of conflict which is actively defined, delineated, and legitimized by implicit social understandings and explicit legal regulations. In other words, war is a social construct, even if military conflict in general is not.
A particularly common example of the Solferino fallacy is the "war is hell" argument, which states that since "war is hell", any effort to regulate it should be abandoned. In this argument, you assume your conclusion at the very beginning: since your understanding of war is that it's hell, then of course by your understanding of it, regulations are pointless. An objector can justifiably respond that this might be what you think war is, but what the mainstream understanding of statesmen construes as war (in jargon, what "international actors" perceive it as) is something with specific limits that does not need to be — indeed, shouldn't be — hell. They can therefore be held accountable to these standards, since it is not a natural force they are attempting to control, but something which they themselves define and initiate.
It is possible to build analytically justifiable arguments about why international humanitarian law is ultimately meaningless: the Solferino fallacy only characterizes specific, but common, arguments that don't hold water.
The term "Solferino fallacy" originates, indirectly, in the 1859 Battle of Solferino, a key turning-point in the development of humanitarian law. More specifically it refers to the 1861 Memoirs of Solferino, a work about the battle which sparked an international movement that culminated with the First Geneva Convention. The Memoirs of Solferino were designed to expose the brutality of war and thus call for laws to be instituted to regulate it. By framing the subsequent debate in these terms, the Memoirs committed the first explicit instance of the Solferino fallacy.
Formally, the Solferino fallacy is an incorrect understanding of war and to some extent an example of the generalized naturalistic fallacy. It is usually incorporated in a type of presupposition, and is also similar to a spotlight fallacy in that it assumes that characteristics that pertain to particular types of military conflict apply to all kinds of military conflict (and hence to war).