What event is generally considered to be the "spark" that started the "war on terrorism"

Most definitely, the attacks perpetrated against the USA on 9/11 was the “spark” or the trigger that initiated what George W. Bush´s administration called “the war on terrorism”, which, right after the attacks, began by invading Iraq and Afghanistan to —so it was publicly defended— overthrow regimes that supported terrorist groups that were behind the attacks (Al-Qaeda at that time) and defeat these terrorist groups themselves. The war in Iraq resulted in the overthrow of Sadam Husein and the destabilization of the country, to this day. The war in Afghanistan, which is still ongoing, has been —so it is argued— focused on defeating Al-Qaeda insurgencies in the region.

demand is the need and oportunity to buy a given good or service.