The last thing President Obama wants is for US to look weak – or to get mired in conflict

This is why the rest of care about America’s choice of president. Never mind Parliament coming back or David Cameron’s national security huddles, what Barack Obama decides is the only thing that will matter on Syria.

It probably comes down to a couple of A4 sheets on the Oval Office desk, one under the heading “downsides”. On it will be two columns and the first – downsides to not striking Syria – will be short. “Look weak, irrelevant, uncaring”. But the second – downsides to striking – will be very, very long.

Let’s start with the people who voted for Mr Obama. They celebrate his disentangling the US from Iraq and Afghanistan. They do not want to get mired in yet another far-away mess. Nor do they want to pay for it. The US army is rationing biros right now, never mind Tomahawks.

Then there is Russia. Imagine this: next Thursday and Friday, Mr Obama will be in St Petersburg for a G20 summit. If torpedoes have already rained down on Russia’s good friend Syria, there will be no pleasantries between him and his host, Vladimir Putin. He will be on Russian soil when relations between Washington and Moscow will be at their lowest ebb since the Berlin Wall came down.

Officials in Washington say any action will be a short, sharp slap, lasting no more than three days. They will call it punishment for crossing the “red line” on chemical weapons not an attempt to dislodge the regime or aid the rebels. But what if Moscow and Damascus are right and it sparks a wider conflagration? What if Syria retaliates, hitting US assets in the region, striking a US ally (Israel) or sponsoring a terrorist attack? What if Iran does any of that for them?

The dilemma is truly acute. The President looks at that single word – “Iran” – and glances back at the first column. That is why looking “weak” is not an option. That will cost America whatever leverage it still has in the region (not much).

Look weak and you encourage Iran to think that it too can cross “red lines” with impunity, like building nuclear weapons and thus becoming the dominant player in the region. Look weak and your influence there is cut to zero. Is that to be your foreign policy legacy?

It all makes the decisions Obama made last week over withholding or not withholding aid to the Egyptian military look simple. And now he must be very thankful he didn’t. Any protracted US military involvement in Syria is only feasible with unfettered access to the Suez Canal, which the Egyptian generals control.