From The Economist
Deterrence is a simple concept: using the threat of force to stop an enemy from doing something. On paper, America should have no trouble restraining Iran thus. The former has a globe-striding army; the latter still relies on warships and fighter jets that predate the Moon landing. In practice, though, Iran has proved devilishly difficult to deter. It is hard to put off insurgents and militias through air campaigns; their goals are attrition and survival, not well-ordered governance, and they are willing to sustain casualties. Full-scale invasion is the only real way to deter them and the history of such interventions is salutary.