||The territory controlled by the Islamic State of Iraq and Syria (ISIS) has grown rapidly since the start of the Syrian Civil War. In 2014, ISIS expanded its control into Northern Iraq, leading to a major humanitarian crisis in the region. This study makes use of night-time light images to detect the ISIS offensive against Iraq in 2014. We developed an algorithm to separate city lights from oilfield lights, based on an urban extent map. The image analysis indicates that all Northern Iraqi provinces experienced a large reduction in city lighting, varying from 16% to 93%. This reduction is especially evident in the three ISIS-controlled provinces, Al-Anbar, Saladin, and Ninawa, where the reduction of city lighting from May to December 2014 was 59%, 50%, and 93%, respectively. Most of the ISIS-controlled cities, including Mosul and Tikrit, experienced a loss of more than 90% in city lighting after being seized by ISIS, while the cities controlled by the Iraqi security forces (ISF) did not lose as much lighting. In contrast, the city lights in Ar Raqqa, Syria, ISIS’s de facto capital, did not show a decline after that region was seized by ISIS. We conclude that the conflict in Northern Iraq has resulted in a major loss of city lighting and that this loss is most likely due to lack of access to the Iraqi electricity supply grid rather than a deliberate ISIS strategy.