So "terrorism" is an "abstract noun" (whatever that is), but "Germany" and "Japan" (names we give to not-precisely-defined groups of people and/or the geographic locales they inhabit, not always even making a clear distinction between the two....) are not?
Can't I use your argument to poo-poo the idea of a war against a "nation", as well? "Pshaw! How can one fight a war against a 'nation'? It's so abstract! So open-ended! Why, the very idea of fighting a war against 'Germany' or 'Japan'! It's preposterous!"
Let's speak realistically for a moment. The war is against radical Islamists. That's who is trying to kill us or convert us, and so that's who the war is against. In order to conform to current sensibilities, we can't refer to the war in these terms, so we call it (some do, anyway - personally I don't care what we *call* the war) a "war on terror". This is understood by most people. However, there are some (like yourself) who like to focus on what we call the war, as if that is paramount, as opposed to what we are trying to do in this war.
No, you can't fight wars against "abstract nouns", you are right. You can't fight wars against "terrorism" and for that matter you can't even fight a war against "Germany". Wars are fought against people: i.e. Nazis. Or, Islamo-fascists. Get it now?
If, on the other hand, the enemy is defined in abstract terms such that he could be anyone and everywhere, then the billions in spending and expansion of federal power can be sold to the voters as necessary and proper.
The War on Terror and, more specifically, the Department of Homeland Security, will move very quickly beyond Islamic militants to the mission originally intended by the Clinton/Gore administration (which actually drafted the plans now being implemented by the Bush administration): the apprehension of domestic anti-government activists.