BARACK Obama's choice to speak to America tonight from the Oval Office is an acknowledgement of the alarm and unease the attack in San Bernardino has engendered.
Amid growing evidence that a married couple who killed 14 people in California last week were at least inspired by the hatred of Isis, President Obama was set to pledge to use "every single aspect of American power" to obliterate it and restore a sense of safety at home. Mr Obama's address will be only the third time he has spoken to the nation from his desk in the Oval Office and the first in five years.
Americans everywhere are asking themselves if even their most mundane routines might now open them to peril.
"The President will ... discuss the broader threat of terrorism, including the nature of the threat, how it has evolved, and how we will defeat it," the White House spokesman Josh Earnest said in a statement. He added that Mr Obama remained convinced that Isis "will be destroyed and that the United States must draw upon our values, our unwavering commitment to justice, equality and freedom, to prevail over terrorist groups that use violence to advance a destructive ideology".
Considerable mystery still surrounds what drove US-born Syed Rizwan Farook, 28, and his spouse, Tashfeen Malik, 29, a native of Pakistan, to open fire on a gathering of employees of the San Bernardino Health Department last Wednesday. The two were later killed in a police shootout.
It does seem clear, however, that the two, possibly to differing degrees, had felt an affinity for Isis and possibly other extremist jihadists who share a mission to attack the West. On the day of the attack, Farook pledged her loyalty to Isis on a Facebook page using a pseudonym and the FBI has said that her husband had previously had some contact with overseas terror groups.
Isis moreover described both of them as "supporters" via internet radio on Saturday and praised what they had done. Among urgent questions now facing investigators: were the two "radicalised" by outsiders or did they "self-radicalise" by digesting the propaganda put out by Isis, notably by social media?
They are also asking if the wife, who arrived in the US on a fiancée visa from Saudi Arabia last year, was in fact the one who led the process, radicalising the husband.
She came from a prosperous Pakistani family and grew up in Saudi Arabia. She returned to Multan, Pakistan, about 100 miles from her ancestral village, in 2007, pursuing a pharmacy degree at Bahauddin Zakariya University. One of her closest friends, Abida Rani, told The Washington Post that Malik changed around 2009, suddenly paying more attention to Islamic studies than to pharmacology. Malik would travel across town, nearly every day, to a school, where she would spend her evenings, Rani said.
"We were like, 'What happened to Malik?'" said Rani, who attended school with Malik for six years. "She became so religious, so serious and so focused on Islamic teachings, and she lost her interest in her studies."
Nothing has yet indicated that the couple was under the direction of Isis. But the fact that they were moved to act by its propaganda has created an entirely new situation for the United States, which had until now felt more or less insulated from the kind of danger faced by European cities, because American Muslims are generally better integrated into society than they are in Europe.
"We have moved to an entirely new phase in the global terrorist threat and in our homeland security efforts," Jeh Johnson, the Secretary of Homeland Security, told The New York Times. Terrorists have "in effect outsourced attempts to attack our homeland. We've seen this not just here but in other places. This requires a whole new approach, in my view."
The only other two occasions Mr Obama addressed the nation from the Oval Office was at the time of the BP oil spill in the Gulf of Mexico and when military operations ceased in Iraq.
His role in this third address is partly as "reassurer-in-chief". Speaking to NBC News the US Attorney General, Loretta Lynch, said the President would tell Americans to "not give in to fear at this time". She said Mr Obama may ask Congress "to review measures and take action" to safeguard national security, without offering specifics.
Most are looking for more than comforting words, but also a clearer picture of what is being done now. "I think what you're going to hear from him is a discussion about what government is doing to ensure all of our highest priority - the protection of the American people," Ms Lynch added.
Hillary Clinton, the Democratic presidential front-runner, called on the US Congress to pass a new military authorisation law to allow intensified attacks by the US military against Isis in Syria and Iraq, calling the current authorisation law, dating back to the post-9/11 period, outdated. She also said the US should do more to counter the use by Isis of social media and the internet to spread its poison.
"We have to have a much more robust air campaign against Isis targets, against oil infrastructure, against leadership," she said on ABC TV. "We need a new update of military authorisation ... to make sure we have every tool out our disposal, number one to destroy their (Isis's) caliphate … and destroy their very effective virtual caliphate that they are using on the internet."
Noting the "very sophisticated propagation" of its message by Isis, Ms Clinton said she saw a three-pronged approach. "We have to fight them in the air, we have to fight them on the ground and we have to fight them on the internet." She, however, ruled out supporting a mass deployment of US ground forces against Isis saying she thought it would make the situation "worse, not better".
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