Rarely is the relationship between cost and benefit as clear as it was last Friday, when Stephen O'Brien, the United Nations undersecretary general for humanitarian affairs, warned the U.N. Security Council that the world is facing its greatest humanitarian crisis since 1945. More than 20 million people in four countries in Africa and the Middle East are at risk and without collective action, "they will simply starve to death." The price tag to save those lives? Just $4.4 billion by July.

The four countries O'Brien identified are Yemen, South Sudan, Somalia and Nigeria. In each case, the cause of the crisis is the same: conflict. In other words, these crises are man-made. Worse, warning signs were evident months ago and the worst of these crises could have been averted.

In Yemen, two-thirds of the population — 18.8 million people — need aid and more than 7 million others are hungry. Those numbers have grown by 3 million in the two months since the start of the year. The roots of this horrific tragedy lie in the proxy war fought by the government — supported by Saudi Arabia and other Sunni states in the region — and Houthi rebels backed by Iran. All parties to the conflict have promised to let aid to civilians go through, but those words have proven empty as military leaders instead deny access to humanitarian aid or use it for their own purposes.