A boy walks on a roof of a house destroyed by recent airstrikes carried out by the Saudi-led coalition in Sanaa, Yemen, on Dec. 29, 2017. Yemen has been locked into a civil war since the Houthi rebels overran much of the country militarily and seized all northern provinces, including the capital Sanaa, in 2014. Saudi Arabia leads an Arab military coalition that intervened in Yemen in 2015 to support the government of President Abd-Rabbu Mansour Hadi after Houthi rebels forced him into exile. The war has killed over 10,000 Yemenis, mostly children, and displaced 3 million others, creating one of the world's worst humanitarian crises. (Xinhua/Mohammed Mohammed)