The killings in Libya will sharpen American concern over what's happening in the Middle East – and who will get the blame in US politics.
Who lost Libya? The US must now worry about losing Syria
Who lost the Arab Spring? After the murder of the US ambassador, Christopher Stevens, and three other American diplomats in Libya, President Barack Obama's performance during the upheaval in the Arab world will inevitably become a highly charged political issue.
So far in the US election campaign, foreign policy has barely figured. When the Republican challenger, Mitt Romney, gave his acceptance speech last month, there was scarcely more than a sentence about foreign policy. Mr Romney seemed to be conceding that the incumbent was unassailable in a field where Republicans generally score highly with voters.
But Mr Romney went on the attack as soon as crowds, protesting at a provocative anti-Islam video produced in California, invaded the US embassy compound in Cairo. He continued after news of fatalities in the US consulate in the city of Benghazi, in eastern Libya, where the ambassador happened to be staying. Mr Romney accused Mr Obama of "apologising for America" and his aides weighed in to blame the president for "weakness".
American liberals have rushed to portray Mr Romney as unfit for the presidency, saying he mishandled the situation by putting partisan advantage ahead of the national interest. Even his running mate, Paul Ryan, has refused to follow him down this track.
Still, a Rubicon has been crossed and the Arab revolutions are now an openly partisan issue. Life will get more uncomfortable for Mr Obama as evidence grows that the assault on the Benghazi consulate was not a mass protest that got out of hand, but a planned operation by jihadists using rocket launchers and heavy machine guns.
The collapse of bipartisanship is hardly surprising. In US politics, no sparrow falls from the sky anywhere in the world without someone being held to account in Washington. And without someone to blame at home, no foreign event is worthy of attention.
This was most clearly demonstrated when China fell to the communists in 1949 after years of civil war. "Who lost China?" was a burning political question throughout the 1950s and beyond. President Harry Truman took some of the blame, but mostly it was hung on General George Marshall, one of the architects of the Allied victory in the Second World War and later a secretary of state who launched the Marshall Plan to revive Europe, for which he received the Nobel Peace Prize.
The fact that Mao Zedong was more ruthless and better organised than the rackety US-backed Nationalists was disregarded. Someone at home had to take the blame for what was seen at the time as America's greatest diplomatic defeat of the 20th century.
Then - as this time - the charge sheet against the incumbent was thin. Speaking to the Foreign Policy website, Rich Williamson, Mr Romney's senior adviser, accused the president of not providing effective leadership in the Arab world. "It's a pattern, and the pattern sees the US with reduced influence, reduced respect, reduced capacity to project its interests."
The accusation of weakness is based on the chimera that George W Bush's so-called "freedom agenda" would have led to peaceful change, not revolution. But Mr Bush, floundering in Iraq and Afghanistan, effectively abandoned any idea of bringing democracy to Egypt and clung to the Mubarak regime. It could be argued, however, that US-funded training in the Bush era of Egyptian grassroots opposition groups may have played a small role in fomenting those very revolutions that Mr Romney now decries.
But the detail probably does not matter. What may stick in the mind of the voter is that Mr Stevens is the first US ambassador to be killed since the administration of Jimmy Carter, who was blindsided and humiliated by the Iranian revolution. And the message from TV is that Arabs' brief honeymoon as members of the Facebook generation is over. Now they have reverted to the 1970s stereotype of people who fire wildly with Kalashnikovs. And that, in the popular imagination, is not a plus for America.
This is not to say that Mr Obama is free of guilt. It was reckless to leave the Benghazi consulate protected by half a dozen Libyan guards of dubious loyalty. Last month, the State Department warned US citizens to avoid Libya for all but essential travel, describing it as place of political violence and militia battles. Only the State Department appears to have failed to heed to its own advice.
Even more seriously, the Obama administration - with France and Britain taking the lead - engineered regime change in Libya without much thought for the future. Libya provided a steady supply of jihadists to fight in Afghanistan, including Al Qaeda's second in command after Osama bin Laden died, Abu Yahya Al Libi, killed by a US drone in June.
His death was confirmed in an Al Qaeda video on the 11th anniversary of the September 11 attacks. This video could have been the signal for the old jihadist's comrades-in-arms to attack the consulate. Ex-members of the Libyan Islamic Fighting Group have plenty of grievances against the US and Britain, which conspired to turn over captured suspects for torture in Muammar Qaddafi's jails before the uprising.
The jihadists are a minority in Libya. In an election in July, secular parties scored a narrow victory. However much trouble they make, the jihadists will not take over, although it will clearly take years for the chaotic state to impose its will on them.
Mr Obama has many questions to answer on how he allowed the embassy to be overrun. But if the charge is that he blundered into regime change without knowing what to do next, then he is hardly alone: Mr Bush did the same in Iraq, and that was a long-planned war of choice.
Libya can sort itself out in time. The real issue is elsewhere: what if, in the course of a long civil war in Syria, the jihadist element in the opposition takes the ascendant and seizes power in Damascus? Then "Who lost Syria?" will be a real question, and the whole world will want to know the answer.
On Twitter: @aphilps