Dead babies are not enough. Nor are primary school children with their throats cut in hideous reprisal for dissent. Nor is chopping off the hands, legs and genitalia of those who happen to be from the wrong religious denomination or ethnic group. All these things are sufficient to provoke international outrage. But it is an impotent outrage.
The violence in Syria has escalated significantly in the past 10 days. The evidence against the regime of President Bashar al-Assad, from 23 separate massacres, has mounted steadily to the point where Amnesty International has accused Syria of "crimes against humanity".
None of this hand-wringing seems to have made an iota of difference. The conflict, which has taken more than 10,000 lives in just 15 months, has become so grave the UN peacekeeping chief in Syria has described it as a civil war and confined his unarmed observers to their bases, suspending all monitoring. Indeed, the condemnation of the international community appears to have stiffened Assad's resolve to stamp out his opponents as quickly as he can.
Victims repeatedly ask foreign reporters why the world is standing idly by. We all know the answer: the Russians have vetoed any meaningful action by the UN. In response, Saudi Arabia and Qatar are providing the rebels with weapons which only escalate the violence. In the US, neo-cons are urging Washington to do the same.
Meanwhile, the European Union has responded with the bizarre gesture of banning luxury imports to the Syrian regime accused of killing thousands of its own people. No more caviar, truffles, pearls, flash cars or big cigars. That'll show them.
So what is the way forward? To answer that, it is important to understand that Syria has become an apocalyptic tapestry woven from some of the most problematic threads of our time. The dominant thread is the desire of the US for stability in the Middle East to protect the supplies of oil on which its economy still depends. The warp to that weft is formed by Russia, China and Iran – the only effective axis of resistance to the world's sole surviving superpower. Woven into that is the unpredictability of Israel, which every US president must treat with gloved respect, given the power of the Jewish vote.
Now a fourth thread is becoming prominent. It is the conflict between Sunni and Shia Muslims. Most of Syria's 22 million population are Sunnis, but they are governed by a Shia minority, the Alawites, to which the Assad family belong. Many of the massacres have been along this sectarian divide. Last week, a Sunni suicide bomber, possibly from al-Qa'ida, hit back by blowing up a vehicle near the Shia shrine of Sayyida Zeinab, to which hundreds of thousands of pilgrims, mainly from Syria's Shia ally Iran, travel each year.
The Sunni-Shia faultline has growing and frightening salience. Iran is Shia. So is the majority in Iraq. So is the Lebanon-based militant movement Hezbollah. By contrast, most of the West's traditional Arab allies are Sunni: Turkey, Egypt, Jordan and arms-supplying Gulf states such as Qatar and Saudi Arabia. But, to complicate matters, al-Qa'ida is Sunni, too, so the West is nervous that those who replace Assad might turn out to be fundamentalist jihadists. There is a very real danger that the violence in Syria could turn to all-out sectarian war. And that could spread through the Arab world.
Which of these threads is easiest to unpick? Despite what is widely said about the intransigence of Russia's support for Assad, the most malleable element could be Moscow. The Kremlin is determined not to lose Syria as the centre of its Middle Eastern sphere of influence. It has $20bn in investments there. It sells 10 per cent of its arms exports to Syria, which gives Russia its only naval base on the Mediterranean.
Moscow, which feels it was tricked into abandoning Gaddafi in Libya, is determined not to make the same mistake over Assad. The West has not been very skilful here. The harsh words of Hillary Clinton last week were typical. She announced that Russia had "dramatically" escalated the crisis by sending attack helicopters to Syria – but then had to admit that it was only sending parts for existing aircraft. The West's rhetoric has reverted to the Crusader indignation used in Iraq rather than the careful language about self-determination in Libya. It has put Russian backs up.
What is needed is a deal which will remove Assad but, as in Yemen, keep reformist elements within his regime sufficiently intact to preserve Russia's strategic interests. Instead of waving a big stick, Obama needs to be dangling carrots. When he and Putin meet at the G20 in Mexico tomorrow, he should offer some good deals when Russia joins the World Trade Organisation in August. He should soft-peddle on public criticism of Russia's political authoritarianism. And he could offer verifiable assurances that America's new missile defence systems are not targeted on Moscow.
Above all, he should pledge that Washington will not take military action against Iran, and will restrain unilateral air-strikes by the Israelis. Chaos along the Russo-Iranian border is a far bigger political and economic threat to Moscow than anything that could happen in Damascus. Putin will drive a hard bargain. He is a hard man. But history shows it is deals with hard men that stick.