The Paris attacks were unprecedented in their scale in France and shocking in their method. The repercussions are likely to be wide and long-lasting.
Here are five areas to watch:
Peace talks to end the Syrian civil war had drifted along for years before a snowballing refugee crisis in Europe this summer and Russia's dramatic entry into the conflict in September gave them new urgency.
Given growing evidence of a Syrian link, the attacks in Paris will hike pressure on world leaders to overcome their deep divisions and solve a problem that is a key source of Islamic extremism.
"If they've done anything they've encouraged us today to do even harder work to make progress and to help resolve the crises that we face," US Secretary of State John Kerry said Saturday at a second round of peace talks in Vienna.
Some of the Paris attackers were overheard telling hostages the attacks were in retaliation for France's bombing of the ISIS group in Iraq and Syria. Paris's air strikes were also referenced by the group in a statement claiming responsibility.
In step with increased diplomatic activity, the attacks appear likely to stiffen Western resolve to continue battlefield pressure against ISIS -- with the risk of being sucked further into the conflict.
ISIS had been on the defensive this week, facing losses to US-backed Kurdish fighters around the town of Sinjar in Iraqi Kurdistan, increased bombing from Russia and the reported death of one of its most infamous executioners, Jihadi John.
French President Francois Hollande sounded defiant in his reaction on Saturday, saying he considered the carnage "an act of war" and promising a response that would be "pitiless".
Europe is facing its biggest refugee crisis since the Second World War as hundreds of thousands flee conflicts or oppression in Syria, Iraq or Afghanistan to seek safety overseas.
Already facing anti-immigration sentiment, the attacks could further complicate efforts by European governments to persuade their populations to accept this burden.
Fears have been regularly stoked by reports that ISIS operatives could be hiding among the 800,000 migrants who have arrived this year, mostly on the shores of Greece and Italy.
Many on the far-right were quick to link the attacks -- so far without foundation -- to the refugee crisis which has distilled fears about the so-called "Islamisation of Europe."
Poland's incoming right-wing government said Saturday it would no longer accept refugees under an EU plan to relocate migrants from Greece and Italy to other countries.
"After the tragic events of Paris we do not see the political possibility of respecting" the EU quota, announced incoming European affairs minister Konrad Szymanski.
Already accustomed to seeing heavily armed security forces guarding schools and synagogues since the Charlie Hebdo attacks in January, residents of the French capital will now face an even more muscular presence.
An extra 1,500 soldiers were mobilised to reinforce police in Paris on Saturday, while European governments held emergency talks to review their security arrangements.
"Last night's attacks suggest a new degree of planning and coordination and a greater ambition for mass casualty attacks," said British Prime Minister David Cameron on Saturday.
More armed police and visible security checks appear inevitable.
Removing national barriers is a key part of the EU project, with Europeans allowed to travel without passports or visas in the 22-nation Schengen zone.
The refugee crisis had already strained this system to breaking point, with a host of countries including Germany and Sweden re-imposing border controls while Austria, Hungary and others are building border fences.
EU President Tusk said this week that "saving Schengen is a race against time" but Friday's attacks have already complicated efforts.
France, Belgium and Germany stepped up border controls, while any indication that the attackers or their weapons had travelled undetected across European borders would add to calls for more scrutiny over people and goods.