Saudi Arabia’s new blockade of Yemen is threatening to exacerbate what the United Nations has deemed the “worst humanitarian crisis in the world.”

For more than two and a half years, Saudi Arabia has spearheaded a brutal military operation in Yemen in support of its exiled government, which was ousted from power after Iranian-backed Houthi rebels seized control of the capital, Sanaa, in 2014.

That military campaign — which is getting significant US funding, logistical support, and arms — has caused enormous suffering in what was the poorest nation in the Arab world even before the conflict broke out. The Saudi Arabia-led operations, which have included various restrictions on Yemen’s airspace and seaports, have caused the deaths of over 5,000 civilians, more than 20 percent of whom are children.

They’ve helped cause the worst outbreak of cholera in modern history. And they’ve contributed to a malnutrition crisis of colossal proportions: Close to 80 percent of Yemen’s population lacks reliable access to food, and the United Nations estimates that 7 million of the country’s population of 28 million people are facing famine.

In recent weeks, the violence has somehow managed to take an even scarier turn, one that could herald a dangerous new phase in the war.

On November 4, Saudi Arabia shot down a ballistic missile that Houthis in Yemen had fired towards Riyadh, the capital of Saudi Arabia. The rebels said they were targeting an airport and that the attempt was a success because it "shook the Saudi capital” despite causing no injuries.

Saudi Arabia has reacted quickly, and harshly. On November 6, it declared the attack “an act of war” by Iran, which backs the Houthi rebels with weapons shipments and advisers. And it tightened its blockade of Yemen, rendering it virtually impossible for humanitarian aid to reach Yemen’s air- and seaports.

Human rights groups immediately condemned the blockade as inhumane, and the UN warned that halting food aid would starve millions. In the past week, Saudi Arabia has indicated that it’s taking steps toward a partial lift of the blockade, but humanitarian organizations say it’s not enough, and want Riyadh to lift the blockade entirely.

A partial lift, they say, will still push at least 3 million more to the brink of starvation, and could result in the deaths of hundreds of thousands of children.

Yemen has been in bad shape for a long time

The roots of the current conflict can be traced back to 2011, when a militant uprising forced the country’s longtime President Ali Abdullah Saleh to step down and hand over power to his deputy, Abd Rabbu Mansour Hadi. Hadi then set about forming a transition government to unite the fractured nation.

Hadi, to put it mildly, didn’t do so. He struggled to rein in a separatist movement in the south of the country, and his government was targeted by the local al-Qaeda affiliate. Militant political groups like the Houthis accused Hadi of shutting them out of the political process. And he presided over a weak economy racked by widespread unemployment, food insecurity, and corruption.

In the fall of 2014, Houthis rebels took over the capital, ousted Hadi, and sparked a civil war.

The war didn’t stay domestic for very long. Hadi enlisted help from Saudi Arabia, which was willing to intervene to counteract Iranian influence in its own backyard. Saudi Arabia and Iran are regional archrivals that fight proxy wars against each other by backing opposite sides in conflicts throughout the region, including in Iraq and Syria.

The Saudi-led military offensive against the Houthis that began in the spring of 2015 has been brutal. Saudi Arabian Crown Prince Mohammed bin Salman, widely known as MBS, considers the war to be a priority in his agenda to ruthlessly confront Iran across the Middle East.

Airstrikes have targeted civilian areas like marketplaces, hospitals, rehab centers for the blind, and funeral homes. Human Rights Watch has documented at least 16 attacks in which the coalition has used cluster bombs banned under international law. Destruction of the country’s infrastructure has caused the spread of easily preventable diseases like cholera. The economy has been brought to a nearly complete standstill.

And the restrictions it has placed on air and sea travel in and out of the country has hampered the flow of vital medical supplies, food, and fuel. The International Aid Group estimates that 130 children die a day due to extreme hunger and disease.

Things are getting worse, and quickly

But the blockade Saudi Arabia launched earlier this month in response to the Houthi ballistic missile launch has made things considerably worse.

International outcry, which has included criticism from human rights organizations and at least symbolic shows of disapproval from US lawmakers, has prompted Riyadh to loosen its grip on Yemen’s borders in recent days. Saudi Arabia has agreed to allow for the reopening of airports and seaports that are controlled by its allies in Yemen.

But three UN agencies — the World Health Program, World Food Program, and UNICEF — have said that it’s not enough. They said in a statement on Thursday that Saudi Arabia’s refusal to lift blockades in airports and seaports controlled by rebel groups is still "making an already catastrophic situation far worse."

Saudi Arabia claims that their crackdown is necessary to prevent arms smuggling to the Houthi rebels. But that crackdown is slowing the entry of humanitarian aid — the limited ports that Saudi Arabia has reopened simply don’t have the capacity to handle the cargo that needs to get into the country to keep ordinary Yemenis from starving.

The UN reports that the blockade could be causing an uptick in the spread of disease already. It also estimates that a partial lift of the blockade will still cause the death of 150,000 malnourished children in the coming months who might have otherwise survived.

Despite international criticism, Saudi Arabia doesn’t feel rushed at the moment, and that’s mainly because throughout this conflict it has had the blessing and support of the US.

The Obama and Trump administrations have provided logistical and intelligence support to the Saudi Arabian-led war effort, and approved billions of dollars in American arms shipments.

President Obama did express at least some criticism of Saudi Arabia’s conduct in Yemen in his final days in office by postponing one key arms deal and dialing back some intelligence sharing. President Trump, by contrast, has supported Riyadh aggressively throughout his time in office and backed some of MBS’s boldest foreign policy moves in the region, such as his controversial decision to lead a blockade against Qatar.

Saudi Arabia knows its international reputation is taking a hit from the brutal way it’s conducting this war. But it also knows it has the quiet endorsement of the most powerful country in the world.

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