Terrorists could have conducted the November 13th, 2015 Paris attacks in any country or city in the world. Further actions can be expected to be undertaken by suicide bombers or perhaps by drones or missiles. Nowhere is immune from such attacks, and no country can provide defense from such attacks. Countries are now faced with a form of world war, the third one in the last hundred years. It is being conducted not by government funded military forces, but by terrorist organizations paid for by individuals from around the world. Containing these actions is far more difficult than is the case of conventional warfare.
Globalization which is pervasive in economic transactions now characterizes conflict. Dangers from the spread of nuclear technology, which have been of concern since WW2, are now surpassed by the spread of other technologies which are far cheaper to acquire and use to deadly effect. Drones, for example, are available in toy stores, can fly at low levels and carry deadly substances. Amazon is experimenting with drone technology to deliver packages.
WW3 began some years ago including events such as 9/11, attacks on various US embassies abroad, bombings in Madrid, London, Paris and throughout the Middle East, assassinations in various countries including Canada. Wikipedia provides a list of deaths and injuries from terrorist attacks to-date in 2015 and for previous years. No single event started WW3, but at some point the accumulation of attacks around the world signal that these are not isolated events. Responding to them will require forceful action, not negotiations leading to some sort of peace treaty. An enemy using youth suicide bombers does not include negotiations in its arsenal.