What do you do when some foreign power wages war against you in an unorthodox manner? Here I am thinking of Ho Chi Minh and his attempt over a quarter century to conquer all of Southeast Asia. Yes, he did try to do that, but for all the blood and destruction he managed only Vietnam and Laos. He was stopped at the Mekong River.
What do you do when some group of imports, that are the residue of some past imperial days, are busily waging war against your culture, your history, and your values?
I am betting that the Goths, Visigoths, Huns, and all the rest never bothered to declare war on Imperial Rome; they simply showed up and went to work with what became, I guess, creative destruction.
Sure as Hell, ISIS (or ISIL or whatever they now call themselves) don't fit into a pattern that might be deemed a formal declaration of War; they simply show up, start lopping off heads, and impose their macabre form of justice.