Wars have traditionally been fought against a defined enemy — another state, with an army, a government and a territory that can eventually surrender, sign a treaty, or simply be defeated in some clearly identifiable, final sense. The War on Terror, the term the United States and its allies used for the broad military and security campaign launched after the September 2001 attacks, was framed differently from the outset: it was declared against terrorism itself, a tactic that can be used by a wide range of different groups operating across many different countries, rather than against a single defined state adversary with clear boundaries and a government capable of eventually surrendering on the tactic's behalf.
Naming a tactic as the enemy removes the usual markers of a war's end
A conventional war against a single state has reasonably clear, if sometimes contested, markers of what victory or an ending actually looks like — the opposing government surrenders, a peace treaty is signed, or the war is otherwise formally concluded through some recognised process. A war declared against a tactic used by many different, often unconnected groups doesn't have an equivalent clear endpoint built into its own framing: there's no single government capable of surrendering "terrorism" on behalf of every group that might ever use it, and no treaty that could bind every future group that might independently choose to adopt the same tactic somewhere else, at some later, unconnected time. This is a large part of why the War on Terror, unlike most previous conflicts with a named, singular state opponent, was never formally declared over through any comparable single decisive event, and different political leaders and analysts have offered genuinely different views over the years about whether, or when, it should be considered to have effectively ended.
A campaign that spanned many countries and forms of conflict
In practice, the War on Terror encompassed a wide range of quite different military and security actions across many countries, from the conventional invasion and prolonged military presence in Afghanistan, to the 2003 invasion of Iraq, which the United States government justified partly by linking it to the broader campaign despite considerable, ongoing controversy over that specific connection, to extensive covert operations, intelligence-gathering programmes and domestic security measures that expanded significantly across many countries well beyond any single battlefield. This sprawling, multi-front character is itself a direct consequence of the underlying tactic-based framing: because the stated enemy wasn't confined to one country's territory or one government's forces, the campaign's own scope readily expanded to wherever the tactic itself, or groups associated with it, were judged to be present.
What we're still unsure about
The basic framing of the War on Terror as a campaign against a tactic rather than a single state, and its resulting sprawl across many countries and forms of conflict, are well documented in the historical and policy record. What remains genuinely, actively contested — a matter of real ongoing historical and political debate rather than one with a settled answer — is how effectively the campaign actually achieved its stated aims relative to its enormous human and financial costs, and how much specific individual decisions, especially the 2003 Iraq invasion, should be understood as a coherent part of the same broader campaign as opposed to a separate, more disputed undertaking that used the same broader justification, a question historians and policy analysts continue to debate rather than one this post can settle.
This sits inside The War on Terror & 21st-Century Conflicts, one of seven topics in Contemporary History, one of seven domains in History, one of seventeen subjects the app can quiz you on.