Luke MacGregor/ReutersThe Sept. 15 terrorist bombing in a crowded London subway station – which injured at least 30 passengers but caused no deaths – was the latest in a string of terrorist attacks in Western Europe in recent years. In mid-August, attacks in Barcelona and the nearby city of Cambrils killed 16 people and injured more than 130 just a few days before an attacker with a knife in Finland killed two and wounded eight others.