About a mile from Paris' iconic Montmartre neighborhood is a spot where very few tourists go. It is a busy overpass that shelters a collection of small, dirty tents. The metro rumbles overhead. Trains pass by below. The residents of this tent camp mostly come from East Africa. Everybody has a different story. But nobody wanted to end up on this bridge, with no toilet, no home and, for now, no future. Mikias, 24, said he was a student in Ethiopia's capital, Addis Ababa, before...