The next purpose is deterrence, which involves convincing those states or coalitions with significant capabilities that actually attempting to employ these assets will result in their destruction and defeat. Unfortunately, Western deterrence concepts are not meaningful or even rational to terrorists and regimes like Iran. Religious extremists who value life in the hereafter more than life on earth are unlikely to be deterred as Moscow's Cold War atheists were, reluctant to toss away their one turn at life. This gaping hole in the deterrence concept requires different strategies against adversaries that are not "rational actors" as we have heretofore understood that term, especially those new threats relying on asymmetric capabilities like terrorism with weapons of mass destruction.

Thus we arrive inevitably at  the issue of pre-emptive or preventative military strikes in self-defence, concepts that are also cousins, not synonyms. In both, however, a nation considering using force before actual military hostilities have commenced against it must be able to justify so doing, morally and politically. The aftermath of the war overthrowing Saddam Hussein has undoubtedly made such self-defence strategies politically less appealing for some, but the underlying logic remains unchanged. Franklin Roosevelt put it well in a fireside chat shortly before Pearl Harbor, justifying his order for US naval vessels to fire first on German ships and U-boats in the North Atlantic: "When you see a rattlesnake poised to strike, you do not wait until he has struck before you crush him." 

Chillingly, that fireside chat was on September 11, 1941. Roosevelt also observed, "It is no act of war on our part when we decide to protect the seas that are vital to American defence. The aggression is not ours. Ours is solely defence."

 Exactly the same logic applies to regimes or terrorists possessing weapons of mass destruction or otherwise poised for aggression. The physical challenges are different, but the risks are obviously even more acute than for Roosevelt because of the far greater destructive power of nuclear, chemical and biological weapons. But if clearing the North Atlantic of hostile vessels was central to US defences prior to World War II, keeping weapons of mass destruction out of the hands of rogue states is undeniably vital today.  

The clear case in point is Iran. Tehran's nuclear weapons programme is unquestionably on the verge of achieving its decades-long quest, being only a year away by Obama Defense Secretary Panetta's estimate, and even earlier in many scenarios. For years, Washington has said that an Iranian nuclear weapon is "unacceptable", but we have then repeatedly proceeded to accept it. Diplomacy and sanctions have failed. Obama's Director of National Intelligence, James Clapper, testified in January: "The sanctions as imposed so far have not caused [the Iranians] to change their behaviour or their policy." While Obama clearly has neither the interest nor the spine to take pre-emptive military action against Iran, there is far less doubt Israel would. Twice before confronted with nuclear weapons programmes in hostile states (Iraq in 1981 and Syria in 2007), Israel has struck decisively before weaponisation was achieved.