That children, even under the worst of circumstances, are able to remain children supplies the world around them with the sense of a future, which is the equivalent of hope. The history of the world is a history of wars — the sieges of Jerusalem, the Mongol conquests, the Mexican Revolution, World Wars I and II, the Taiping Rebellion, the Armenian genocide, the Napoleonic Wars, the Dungan Revolt, the Vietnam War, the Seven Years’ War, the 30 Years’ War, the Roman civil wars, the Chinese Civil War, the American Civil War, the Chilean civil wars, the Liberian civil wars, the Spanish Civil War. These and countless others are usually fought by young adults and overseen by old men, and it is their experiences that historians tend to consider. But children (and their mothers) are present, too. Those who are not killed wind up displaced, surviving in camps and bombed-out villages, where by their mere presence they contribute to the continuance of humankind. Less obvious than the biological fact of this is the psychological one. If there were no children, would the adults of a refugee camp have the will to endure?

With so many wars behind us, so many around us and so many (we must assume) ahead, it begins to seem as if we are wholly dependent on children to provide the world with whatever minimum fuel of promise is required for it to keep turning. The kids have no choice in the matter. They cannot be any other place or any other age. They can only be resilient, miraculously so. Where does this capability come from? Are children able to recover so quickly from adversity because of their inexperience, because they don’t realize quite how bad things really are? Or is it because they have a greater capacity than adults to live entirely in the present, to lose themselves in a game of soccer or tag? Or perhaps resilience is a concept supplied by adults, who would like to believe that children will overcome the terrible experiences we foist upon them. Much of the research suggests that kids who encounter repeated or sustained trauma and overwhelming stress are likely to suffer outcomes that range from debilitating depression and post-traumatic stress disorder to heart disease and diabetes. Though these children may furnish their ruined cities and shattered families with a little hope, they won’t emerge unscathed.

In our current crisis, nearly 30 million children worldwide have been driven from their homes by war and persecution. Media coverage has lately focused on the Syrian dimension of this tragedy (in part because of the fate of one refugee child, Aylan Kurdi, the Syrian toddler who drowned crossing the Mediterranean in September). But these 30 million girls and boys are from all over — Afghanistan, Iraq, Eritrea, Libya, Nigeria, Honduras, El Salvador, Myanmar, Bangladesh. This week’s issue of the magazine has been built around portraits of three of them: an 11-year-old boy from eastern Ukraine named Oleg, a 12-year-old Syrian girl named Hana and a 9-year-old South Sudanese boy named Chuol. All three have seen their homes destroyed; two have lost family members. Yet they carry on. Their stories are told in a multimedia documentary project, comprising a feature story, interviews, three photo essays and, for the first time in the history of The New York Times, a virtual-reality film.