The attacks can happen anywhere: In a holiday market outside a historic church in Berlin, on the street during a national holiday in France, in an airplane over Scotland or during the first day of school in a small town in Russia.

[French police come under fire in Paris; three reported shot]

Sometimes, a plot fails – a detonator malfunctions or authorities make an arrest – and no one is injured.

Other times, an attack leaves a trail of death and destruction that can shake a nation, even a continent. But these incidents are nothing new.