Christopher Johnston is a fellow at Georgetown University's Institute for the Study of Diplomacy and a graduate of the Royal Military College, Duntroon.

President Obama should choose his next battle carefully. Meaningful intervention in Syria would sap the military strength and political capital he needs to stop Iran from readying a nuclear bomb.

The most important strategic consideration for the US in Syria should be the regional ambitions of Iran. President Obama could probably order limited airstrikes against the Assad regime at little cost to US interests, but a more significant intervention would erode America's military deterrent against the Ayatollah's nuclear program. That program continues uninterrupted by the election of Iran's agreeable new president.

It's easier to talk America into war when the villain comes straight from central casting. Since the urbane President Rohani replaced the firebrand Ahmadinejad, a US attack on Iranian nuclear facilities seems increasingly unlikely. Mr Rohani's inauguration does present the opportunity for renewed dialogue, and maybe even some progress. Yet amid this enthusiasm, the centrifuges spin patiently beneath the mountain redoubt of Fordow and at Natanz.

Iran's stock of low enriched uranium has more than trebled during the Obama Administration while its supply of 20%-enriched uranium will probably reach a critical mass this year. Once that occurs, the dash to weapons grade uranium could be accomplished within one or two months.

Obama was foolish to stake his nation's credibility on a 'red line' in Syria. The liberal wing of the Democratic Party is encouraging détente with Tehran, while at the same time urging Obama to step up the fight against Assad. This is a mistake.

Nor should Syrian intervention be considered necessary to counter Iran's regional influence. The Ayatollah's ultimate prize is not control over Syria. He could use an atomic weapon to menace the entire region.

In fact, military action against Iran's only regional ally would most likely fuel Persian paranoia and encourage the Supreme Leader's defensive brinkmanship. Khamenei is still waiting for the right moment to sprint for the nuclear finish line. He could not imagine a more fortuitous circumstance than America on the ropes in Afghanistan and decisively engaged in Syria amid rising turmoil throughout the Middle East.

Impoverished and isolated, Iran seems an unlikely adversary for the world's only superpower. Yet a nuclear-armed Iran would present a far greater threat to the US than an enfeebled Assad regime. Successive governments of the Islamic Republic have sponsored violence against Americans, from the 1983 Beirut bombings to the bizarre 2011 plot to assassinate the Saudi Ambassador and attack civilian targets in Washington, DC. Nuclear weapons would further embolden Iranian action, including in Syria.

Iranian defiance would also unravel the slender threads binding signatories to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT). Nuclear disarmament is perhaps the most idealistic objective of President Obama's foreign policy. Could he countenance the failure of the NPT, the only credible international concord preserving a fragile status quo? This could prove the most dangerous legacy of President Obama's second term.

Iran has suspended enrichment only once, in 2003, fearing imminent attack from America. Since then Khamenei has concealed Iran's nuclear progress behind articulate spokesmen such as Mr Rohani. Chief negotiator Rohani allegedly duped Britain, France and Germany into friendly dialogue in 2005 to conceal illicit works in Isfahan.

President Obama can afford to open perhaps one more front in the Middle East. His vow to 'do what we must' is probably the last hurdle preventing Iran's dash for the bomb. Certainly, diplomacy in Iran will fail without the imminent threat of US force. There is no successful precedent for accommodating an Ayatollah.

With that in mind, President Obama must conserve finite US military resources to guard against the greatest strategic threat in the region, Iran. This consideration must run deeper than moral posturing. Punitive airstrikes are one thing. Meaningful intervention in Syria is another question entirely. As the region slips further into chaos, a weakened, but surviving Assad regime could represent the lesser of two evils.

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