As Saudi Arabia played at the World Cup, the country launched a massive attack on Yemen

Millions of eyes were on Saudi Arabia Thursday morning as its team played the opening match against Russia in the World Cup.

Twitter was full of fans tweeting support for the two sides, commenting about the game, and sharing snarky memes based on an image of Russian President Vladimir Putin teasing Saudi Crown Prince Mohammed Bin Salman after Russia scored the first goal in the match.

But while the crown prince laughed and joked with Putin in Moscow’s Luzhniki Stadium, 2,800 miles away, Saudi-led forces were launching one of the largest attacks on Yemen since the war began in 2015.

And international monitors are warning that the assault could result in the deaths of tens of thousands of people and create a humanitarian catastrophe that dwarfs anything we’ve seen so far.

Here’s why: The Saudi assault, which began Wednesday, is targeting the Yemeni port city of Hodeidah, currently held by Iranian-backed Houthi rebels. As much as 80 percent of the food, medicine, fuel, and other aid that enters the country comes in through the Hodeidah port.

Which means that not only are the estimated 250,000 people who live near the port at risk of losing their lives, but millions of others who depend on the aid funneled through the port will be also be endangered.

“It is the lifeline of the country,” Lise Grande, the top United Nations humanitarian official in Yemen, told the Washington Post. “If you cut that port off, we have a catastrophe on our hands.”

The three-year war has ravaged Yemen, which was already one of the poorest countries in the region.

The numbers are staggering: 10,000 people have died since the conflict began; 2 million have been displaced. The country is suffering from the worst cholera outbreak ever recorded in the history of the world. And more than 8 million people are at risk of starving to death.

As Vox’s Zeeshan Aleem wrote back in November, the Saudi war on Yemen, which started in the spring of 2015, “has been brutal.” He explained:

Airstrikes have targeted civilian areas like marketplaces, hospitals, rehab centers for the blind, and funeral homes. And as of last year, Human Rights Watch has documented at least 16 attacks in which the coalition has used cluster bombs banned under international law. 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.

And things have only gotten worse since then. Now Saudi Arabia is launching its biggest attack yet — against the most critical port city in the country.

Saudi Arabia isn’t fighting this war alone. The war is actually being fought by a coalition of Arab states that includes the United Arab Emirates and Bahrain, all led by Saudi Arabia.

But there’s more: The United States and Britain have also supported the Saudi war effort. The US, in particular, has sold military equipment, refueled planes, and provided intelligence to the Saudi-led coalition.

Back in March, the US Senate voted down a resolution to end US support for the war in Yemen, while President Trump met with Saudi Crown Prince Salman at the White House. “Saudi Arabia has been a very great friend and a big purchaser of equipment and lots of other things,” Trump said at the time, adding that the relationship between the two countries “will probably only get better.”

And as recently as Tuesday, the Wall Street Journal reported that the US was sharing intelligence with the Gulf countries involved in the Hodeidah airstrikes. “The intent is to minimize the number of civilian casualties and the harm to critical infrastructure,” a US military official told the Wall Street Journal.

The UN, meanwhile, is actively working to stop the bombing campaign on Hodeidah. On Thursday, the UN Security Council held a two-hour emergency meeting to discuss the situation. Russia’s UN Ambassador Vassily Nebenzia, the current president of the UNSC, told reporters that the council was “united in their deep concern about the risks to the humanitarian situation” and “reiterated their call” for the Hodeidah port to stay open.

But as of Thursday afternoon, hundreds of civilians were fleeing the area as bombs rained down around the port city. It’s anyone’s guess how long this assault will last, or what will be left in its wake.