The United States and Britain will “not be cowed” by Islamist militants in Iraq and Syria who have killed two American journalists, the leaders of both nations said Thursday. Ahead of a NATO summit in Wales that began Thursday, President Barack Obama and British Prime Minister David Cameron published a joint article in the Times of London saying they will “confront” the militant group Islamic State of Iraq and Greater Syria (ISIS), and calling for international action to address security threats around the world. “If terrorists think we will weaken in the face of their threats they could not be more wrong,” they wrote.