The world's largest humanitarian crisis is not in Syria. It's in Yemen.

So says the United Nations, highlighting the grave consequences of a conflict that has left 18.8 million people in need of humanitarian or protection assistance, including 10.3 million who require immediate assistance to save or sustain their lives.

READ MORE: 'Yemen was once a place of joy and spirit'

The war and its economic effect are driving the largest food security emergency in the world: more than 17 million people in Yemen are currently food-insecure, of whom 6.8 million are severely food-insecure and in need of immediate aid.

The conflict has been going on for years but became especially violent in March 2015 after the involvement of a Saudi-led coalition.

More than 10,000 people have been killed and three million displaced from their homes since February 2014, according to the UN.

READ MORE: UN - Yemen faces world's worst cholera outbreak

On July 2, the World Health Organization reported that a cholera outbreak in the country has claimed 1,500 lives, warning that it could infect as many as 300,000 people by the end of August.

With no sign of the conflict abating, it is likely that the humanitarian crisis in the Arab world's poorest country will only get worse.

So how did it all start? Watch our video above to find out more.