World leaders and politicians rallied around France in the wake of Friday night's attacks in Paris, the worst terror incidents to take place in Western Europe in more than a decade. At least 129 people were killed and hundreds more injured, some critically.

French President Francois Hollande deemed the violence, carried out by three sets of assailants, apparently under the aegis of the Islamic State, "an act of war." The attacks were planned in and outside France, and involved foreign and French nationals, authorities said.

President Obama described the killings as "an attack not just on the people of France," but "on all of humanity and the universal values that we share." German Chancellor Angela Merkel said the terror "hits all of us," and merited a collective response from the international community.

But what that response should be is unclear. France's tough-talking prime minister, Manuel Valls, said French authorities would beef up surveillance and security at home, adding that "French nationality should be taken away from those who flout the values of the Republic" — a move that would present a break with current legislation, reports Politico Europe.

Valls also stressed the need to "annihilate" the Islamic State, an extremist organization whose positions in Syria and Iraq are already in the crosshairs of a U.S.-led bombing campaign. France has conducted sorties against the Islamic State.

In a statement, Sen. Dianne Feinstein (D-Calif.), vice chairman of the Senate intelligence committee, said current efforts were inadequate to counter the jihadists, and that the United States ought to do more to "directly" influence the fight.

"It has become clear that limited air strikes and support for Iraqi forces and the Syrian opposition are not sufficient to protect our country and our allies," she said. "This is a war that affects us all, and it’s time we take real action to confront these monsters who target innocent civilians."

In an article in Foreign Policy, James Stavridis, former supreme allied commander of NATO, argued for a muscular response from the military alliance's 28 member states, suggesting the "fundamental purpose of a NATO mission should be to defeat the Islamic state in Syria and destroy the infrastructure it has created there."

The necessity for such a response, it can be argued, is mandated by Article 5 in NATO's founding treaty, which obliges member states to defend a fellow member state under attack. But precedent — most recently seen in NATO's cautious approach to defending Turkey, which has seen both Islamic State terror, Syrian anti-aircraft fire, and the trespassing of Russian fighter planes — would suggest a more calibrated response. Article 5 has only been activated once: after al-Qaeda attacked the United States on 9/11.

The West's ability to "annihilate" the Islamic State is also complicated by the context of the war against it. In Syria, France has launched hundreds of sorties against the Islamic State, which has been fighting the government of President Bashar al-Assad. At the same time, France, perhaps more so than any other European nation, has been particularly vocal about the need to oust Assad.

Not surprisingly, as my colleague Hugh Naylor reported, Assad had choice words on Saturday for French authorities, who he claimed had helped fan the flames of extremism.

"Wrong [policies] adopted by Western states, particularly France, toward events in the region, and its ignorance of the support of a number of its allies to terrorists are reasons behind the expansion of terrorism," Assad was quoted as saying by Syria's state news agency, SANA.

But Assad’s opponents pin the spiraling crisis in Syria, which enabled the emergence of the Islamic State, on his bloody-minded refusal to step down, and his regime’s continued attacks on civilian centers. Syria’s future was the subject of a high-level meeting in Vienna. Foreign ministers from nearly 20 nations convened on Saturday in the latest effort to thrash out a plan for a political solution to the country's grinding, relentless civil war, which is now in its fifth year, has displaced half the country's population and led to the deaths of more than 250,000 people.

They partially agreed on a plan to force most of Syria's warring factions to the table, and start a process that could lead to a meaningful cease-fire as well as a new, inclusive Syrian government. But huge sticking points remain, not least over Assad's role. His allies like Russia and Iran want to see his regime endure, while opponents, from the Sunni Gulf states to Turkey to Western countries like the United States and France are adamant that he must eventually exit.

The attacks in Paris perhaps reduce the pressure on Assad even further.

"There is no justification for terrorist acts," said Russian Foreign Minister Sergei Lavrov, standing next to Secretary of State John Kerry. "And no justification for us not doing much more to defeat ISIS and al Nusra and the like," he added, referring to another name for the Islamic State as well as an al-Qaeda proxy fighting in Syria.

"It doesn't matter if you are for Assad or against him," Lavrov said. "ISIS is your enemy."