Police in Italy, Britain, Norway and Finland on Thursday arrested 15 members of a jihadist network linked to Islamic State and al Qaeda who are suspected of planning attacks and kidnappings targeting EU diplomats, officials and journalists to bargain for the release of jailed colleagues. Italy’s anti-terrorism chief Giuseppe Governale, announcing the details of “Operation JWeb,†called it “the most important international police operation in Europe in 20 years.†The detainees included major names on the global jihadi network, foremost among them Mullah Krekar, an Iraqi Kurd who co-founded Ansar al-Islam, an armed Sunni Islamist group linked to al Qaeda...