The hijackings and crashes were crimes, very serious crimes. The procedure for dealing with crimes is to bring the responsible persons to trial, and punish those convicted through the judicial system. War is armed conflict between nations. There are no trials, just military force. In this case, at least as far as is reported, no other nation is responsible for these crimes. They are acts of individuals, or groups, that are not nations. Yet many politicians persist in talk of war. Not just in simile or metaphor, like the "war on poverty" or "war on drugs" but actual military force, bypassing the judicial process.
If it turns out that another nation refuses to follow established procedures for extraditing individuals in their country who are indicted for crimes in other countries, then perhaps war would be appropriate. But we should not bypass the judicial process and resort to military force in the first place.
Resorting to war is dangerous. First, the government is given extraordinary powers in times of war to curtail civil liberties, to engage in searches and seizures in the name of national security, to alter spending priorities and so on. Once our freedoms and our established procedures are given up, they will be difficult or impossible to regain. Second, it will wreak havoc on world order, a sure recipe for unending war and strife, if countries begin to invade and attack individuals and groups in other countries whom it suspects of committing crimes.
It makes more sense for nations to behave in a judicial, and judicious, fashion, capturing and punishing actual criminals, not turning to war as a first resort.