War is at a tipping point: we're passing from the age of industrial warfare to a new era of computerised warfare, and a renewed risk of great-power conflict. Humanitarian response is also evolving fast--'big aid' demands more and more money, while aid workers try to digitalise, preparing to meet ever-broader needs in the long, big wars and climate crisis of the future. This book draws on the founding moment of the modern Red Cross movement--the 1859 Battle of Solferino, a moment of great change in the nature of conflict--to track the big shifts already underway, and still to come, in the wars and war aid of our century. Hugo Slim first surveys the current landscape: the tech, politics, law and strategy of warfare, and the long-term transformations ahead as conflict goes digital. He then explains how civilians both suffer and survive in today's wars, and how their world is changing. Finally, he critiques today's humanitarian system, citing the challenges of the 2020s. Inspired by Henri Dunant's seminal humanitarian text, 'Solferino 21' alerts policymakers to the coming shakeup of the military and aid professions, illuminating key priorities for the new century. Humanitarians, he warns, must adapt or fail.