Mosques burn and a filmmaker is murdered in a culture clash between Muslims and Christians in the Netherlands. A series of bombs tear apart four commuter trains in Madrid killing 191 people and wounding 1800. Al Qaeda terrorist cells are uncovered in the U.K. Germany Italy and Spain. FRONTLINE investigates the new front in the war on terror: Europe. Now home to 20 million Muslims - which some call "Eurabia" - the continent is a challenge to intelligence services on both sides of the Atlantic exacerbated by political divisions over the Iraq War.