Our world produces enough food to feed all its inhabitants. When one region is in the grips of severe hunger, global humanitarian institutions, though often cash-strapped, are theoretically capable of transporting food and averting catastrophe.

But this year, South Sudan has slipped into famine, and Yemen, Nigeria and Somalia are on the verge of their own. Famine threatens 20 million people - more than at any time since World War II. As defined by the United Nations, famine occurs when a region's daily hunger-related death rate exceeds 2 per 10,000 people.

The persistence of such severe hunger, even in inhospitable climates, would be almost unthinkable without war.

Each of these four countries is in a protracted conflict. While humanitarian assistance can save lives in the near term, none of the food crises can be solved in the long term without a semblance of peace. The threat of violence can limit or prohibit aid workers' access to affected regions, and in some cases, starvation may be a deliberate war tactic.

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War and famine along the White Nile, in South Sudan

In February, the United Nations declared a famine in South Sudan's Mayendit and Leer counties. It was the world's first famine declaration since 2011, in Somalia.

But even in these two counties, more people die every day from bullets than from empty stomachs or disease. The state the counties are in, Unity, has seen some of the worst violence since South Sudan became an independent country in 2011.

Unity is the home state of Riek Machar, a former vice president and the leader of a rebel army of mostly ethnic Nuer people that has been locked in violent confrontations with South Sudan's army, controlled by President Salva Kiir of the Dinka ethnic group, since 2013. Kiir's army and allied militias have swept through Unity time and again, razing and burning entire villages, slaughtering and raping as they go. Thousands of people have drowned in the state's rivers and swamps as they fled.

Those rivers and swamps would otherwise provide Unity's people with abundant fish and water for irrigation. But relentless war renders just about all aspects of daily life unsafe, with people too afraid to leave home, fish, plant or trade. Many are eating grass and water lilies just to survive.

Both the rebels and the government have made it difficult for aid workers to reach the most-affected counties. The Washington Post's Africa correspondent, Kevin Sieff, recently reported on the government's obstructionism.

Sieff described how, at more than 70 checkpoints on the road between Juba and Unity state, soldiers would often demand bribes or food from aid workers, and how the government refuses to let the United Nations operate flights that could drop food supplies over at-risk areas. Dozens of aid workers attempting workarounds have been killed in the war's crossfire.

The United States and others in the U.N. Security Council have proposed an arms embargo to limit the South Sudan government's capacity for violence. But when it came to a vote in December, more than half of the council members, including China and Russia, abstained. Neighboring African countries have discussed a coordinated armed intervention, but that has not garnered much support.

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Civil war leaves Yemen splintered and under siege

Since 2015, Yemen has been in a civil war. The fighting has divided control of the country along sectarian and ideological lines, and killed more than 10,000. It has also decimated Yemen's economy.

Yemen was fragile before the war, but its currency, industry, transport infrastructure and public services have all but been destroyed in the past two years. Millions are jobless, and food and fuel prices have shot through the roof. An estimated 17 million people, or 60 percent of the population, are in need of urgent humanitarian assistance; about 7 million are living day-to-day, enduring until they wither away.

The physical destruction has mostly been the work of the Saudi-led coalition - advised and supplied by the United States, Britain and others - that has sided with Yemen's Sunni president against the Houthis, a Shiite militia that controls the capital, Sanaa, and much of the country's western coast.

One key piece of infrastructure that the coalition has made near-inoperable is the Houthi-controlled port of Hodeida, Yemen's largest and most vital. Almost 90 percent of Yemen's food is imported, and most of it came through Hodeida. Saudi ships are enforcing a near-total blockade of the port, arguing that they can't risk arms smuggling, even though the United Nations inspects each ship on arrival.

Should the coalition move to take Hodeida's city and port militarily, it could shut off what trickle of food is headed to Sanaa and other highly populated inland areas, triggering a famine, according to aid agencies. Coalition officials have argued that if they took the port, they could ensure the passage of aid without worrying about arms smuggling.

Either way, vast swaths of Yemen are under constant bombardment from the coalition. Three-quarters of the residents of the south-central city of Taiz and its surrounding areas, for instance, are facing an emergency food shortage because the area is effectively inaccessible.

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Eight years of terror create a nightmare in northeastern Nigeria

Boko Haram's bloody reign of terror in northeastern Nigeria's Borno state has been so intense over the past few years that aid groups have struggled to even enter the region. Reliable data on hunger is limited. Some aid workers speculate that Borno may have already passed through periods of famine, or may be in one now.

The fighting has displaced more than 3 million people and left a previously fertile region desiccated and barren. Vast camps have sprung up within Nigeria, as well as across the border in Niger and Cameroon. The population of Borno's relatively safe capital, Maiduguri, has doubled because of the influx, and the city is a hub for disease. Tens of thousands of Nigerians, meanwhile, have set their sights north toward Libya and, ultimately, Europe, attempting an expensive and dangerous journey that many do not survive.

Almost as many as those who have fled Boko Haram-controlled areas have stayed behind. They are most at risk of starvation, because their villages are inaccessible to outside aid.

Nigeria's military, even in cooperation with neighboring countries and U.S. and British advisers, has proved sorely inadequate in rooting out the insurgency, although they have made some progress. When they have succeeded in liberating towns and villages from Boko Haram, they often find residents eating grass and insects because that's all that is left.

The United Nations has warned that half a million children in northeastern Nigeria are so severely malnourished that 75,000 could die by June. A growing measles outbreak in the region could transform into an epidemic, too.

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A drought in Somalia, a land awash with guns

Six years ago, more than a quarter of a million Somalis died in a famine. The rains have now failed for two consecutive years in parts of the country, and there are growing fears of a repeat catastrophe. But droughts are common in Somalia and do not always result in famine. The common link between 2011 and today is the continued presence of al-Shabab, an armed group closely linked with al-Qaeda.

While al-Shabab has lost ground since 2011, the famine risk in Somalia is concentrated in rural areas in the country's south, where the group is still strong. That is because the militant group severely restricts the movement of locals who may be in search of scarce food and water. They also restrict access to aid.

As wells have dried up, people have resorted to drinking any water they can find, even if it is dirty. Consequently, a growing cholera epidemic competes with the deepening food shortage.

Yet Somalia, surprisingly, is where there is the most optimism about averting a famine. Al-Shabab has recently given assurances that it will permit freer movement of people. The group's power has also declined significantly, meaning that climatic conditions contribute more to Somalia's crisis proportionally than the others.

And while a drought can leave a nation reliant on aid, that is ultimately an easier problem to solve than war.

At this time of unprecedented need, the world's biggest supplier of humanitarian relief is getting ready for a major cutback.

Humanitarian aid makes up a tiny fraction of U.S. government spending - less than 1 percent - but the Trump administration's proposed budget would eliminate much of it. The State Department and the U.S. Agency for International Development could see their budgets reduced by more than a third. U.S. funding to the United Nations might drop by more than half.

The United Nations had sought $4.4 billion by the end of March for emergency hunger relief operations but has raised barely a fraction of that.

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Sources: Conflict data via IHS Jane's Terrorism & Insurgency Center. Food insecurity data via fews.net. Displaced persons camp data via UNHCR and immap.org via mal-khameri@immap.org. Yemen territorial control via criticalthreats.org. Drought data via NASA Earth Observatory.