Iran's assassination plot against Saudi Arabia's ambassador in Washington has produced widely varying reactions. Curiously, some US "experts" on Iran questioned the veracity of the Obama administration's statement of the case, arguing it was uncharacteristic for Tehran to use intermediaries like Mexican drug cartels rather than old standbys like Hezbollah.

Apparently, under this sceptical view, the official terrorist rule book prohibits creativity and innovation. And at least some intelligence community denizens were miffed that DEA and FBI gumshoes uncovered the conspiracy, not "real" intelligence professionals. In reality, the sceptics are simply revealing their own blindness to Tehran's maliciousness.

In fact, for the Tehran regime, the idea of killing a senior Saudi official, humiliating America by doing so on our own soil, and throwing us off balance by using an extensive foreign criminal network we have been unable to neutralise, is par for the course. The only surprise is who in Washington was surprised by Iran's increasing brazenness.

Administration officials, for example, reacted with incredulity to potential violations of international norms protecting diplomats. Iran's nuclear weapons program, its deadly attacks on US troops in Iraq and Afghanistan and its role as the world's central banker of terrorism had not persuaded President Obama to take strong and decisive action against Tehran, but threatening diplomats raised his pulse rate.

Nonetheless, precisely because it was this president who believed the evidence so overwhelming that criminal prosecutions ensued, we must consider these allegations to be truly serious. Career justice department prosecutors, for whom professional integrity and ethics – not to mention their conviction records – are at stake, believe they can prove guilt beyond a reasonable doubt. This is not a Mad Magazine "Spy v Spy" story.

Having previously given Iran the benefit of every doubt, Obama is now making the public case, a most unlikely scenario unless the evidence was overwhelming (not only what is already public but that which remains undisclosed). If this prosecution were to collapse, it would be politically devastating for Obama.

Ironically, therefore, his policy response betrays the same blindness of those sceptical of the plot itself. The troubling but unavoidable reality is that Iran's regime is increasingly brazen because it sees the United States, under current management, as weak, fickle and inattentive. Unfortunately, the mullahs may be on the verge of being proven correct, yet again, by the president's flaccid response to this diplomatic version of Chicagoland's St Valentine's Day massacre.

President Obama's threat to "apply the toughest sanctions", to Iran will simply convince Tehran of his lack of real seriousness. First, additional sanctions by just Washington and Brussels will cause only incremental increases in Tehran's costs of doing business, and will be evaded just as existing sanctions already are, with assistance from Russia, China, Venezuela and others. Second, the likelihood of obtaining truly significant new sanctions from the UN security council is doubtful. Expect pious pronouncements from Moscow and Beijing, centers of refined due process and independent judiciaries, about the rights of defendants and presumptions of innocence. Time enough for sanctions, they are rehearsing to say, once the judicial process has concluded, and one or more defendants are actually found guilty. Good luck waiting for that.

In fact, by focusing so intensively on just the assassination plot, Obama is ignoring the overwhelming broader implications. Iran's scheme is far more important for what it reveals about the nature and character of Tehran's rulers than the particulars of one gambit, however abhorrent. A terrorist-sponsoring regime capable of putting the occasional ambassador in mortal peril is sufficiently rabid that its likely acquisition of nuclear weapons in the very near future will magnify its threat to truly existential proportions, at least for small countries nearby. And it is the nuclear weapons that should truly concentrate our attention, since they will put us all in mortal peril.

We must abandon the mirages, to which Obama still clings, that Iran might negotiate an acceptable "solution" to its nuclear weapons program, or merely that economic sanctions will somehow force Iran to negotiate. No wonder the Iranian regime mocks us for weakness and willful blindness. Even a threatened attack on our soil, which could have killed hundreds of Americans, has not been enough to spur Obama into decisive action.

The unpleasant reality is that the only alternative to a nuclear Iran is to break Tehran's program through the targeted use of military force, either by Israel, the United States or both. This is, to be sure, a risky, unpleasant and unattractive option. It is, nonetheless, far preferable to the only existing – and rapidly approaching – alternative, which is Iran with nuclear weapons. Although unwilling to say so publicly, Saudi Arabia and other Arab states have long privately hoped for leveling Iran's nuclear program. With the Iranian assassination plot now public, they might even smile publicly.