Possible consequences

Syrian peace talks

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.

Western military involvement in Syria

Some of the Paris attackers were overheard telling hostages the attacks were in retaliation for France's bombing of the Islamic State 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 Islamic State -- with the risk of being sucked further into the conflict. 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".

European refugee crisis

Europe is facing its biggest refugee crisis since the Second World War. 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 IS operatives could be hiding among the 800,000 migrants who have arrived this year. 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." In early signs, Poland's incoming right-wing government yesterday said it would no longer accept refugees under an EU quota plan.

'Borderless' Europe

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 and 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.