The only credible option for significantly delaying the Iranian nuclear program would be a bombing campaign. But who imagines that President Obama will do what his predecessor wouldn't — namely, unleash a war against the ayatollahs? The use of force, despite the bluster from Washington about "all options" being "on the table," is not a credible threat (except from Israel), and the mullahs know it.
In short, Western policymakers have implicitly made the same assumption today that their predecessors made in the 1930s, 1940s and 1990s: that an immediate war, even one fought on favorable terms, is to be feared more than a looming cataclysm that is likely to occur at some indefinite point in the not-too-distant future. That was the right decision to make with Stalin's Russia; it was tragically wrongheaded with Hitler's Germany and the Taliban/Al Qaeda.