The video of five police officers subduing and then killing a man, despite his pleas that he could not breathe, could have come from many countries around the world. But the killing of Eric Garner in New York City, like that of Michael Brown in Ferguson, Missouri, last summer and of 12-year-old Tamir Rice in Cleveland in November, reflect the heightened risk that young African-American men face at the hands of the police in many US cities.