Feat your eyes on the following picture lifted from the BBC’s story covering the manhunt for Dzhokhar Tsarnaev:
That picture shows very succinctly why terrorism works. For the cost of some explosives and a couple of pressure cookers two individuals have managed to cost the state what must be approaching millions of dollars. The streets of Boston are crawling with expensive police officers, wearing expensive armor, driving expensive armored personnel carriers, and carrying expensive weaponry.
Terrorism works because it exploits basic economics. For very little cost a handful of individuals can excise a heavy toll from the state. Warfare, ultimately, is a competition of economies. The loser of a war is the one who no longer has resources to continue. They lose because they run out of soldiers, guns, ammunition, food, medical supplies, effective countermeasures against the other army’s weapons, etc. Fourth generation warfare tactics, such as terrorism, work because they’re too expensive to counter militarily.
An improvised explosive device, like the bombs used in Boston, can injure and kill multiple people with ease. When killing a soldier you not only kill a person but you also wipe out the resources poured into his training and unrecoverable equipment. If you injure a soldier you not only remove a fighter off from the field, possibly permanently, but you also cost his military resources in medical care. Killing an innocent bystander can also cost the other military morale. Vietnam was an example of this. By killing Vietnamese civilians the United States military generated a great deal of hatred back home and that hatred, often aimed at soldiers, eroded the morale of fighting men.