Military intervention by Saudi Arabia in the conflict in Yemen is ringing alarm bells around the Middle East and the world.
Yemen, a historically impoverished and lawless Middle Eastern state, has been the poor relation to its oil-rich neighbors for decades. Its population, subject to a series of puppet governments since the British were driven out in 1967, is malnourished and mismanaged. It's not hugely surprising that guerrilla fighters from other parts of the Middle East, including some from Al-Qaeda, have often hidden out in its countryside.
Here, we take a look at the background to the current conflict and why it is so important to the region.
Take a look at where Yemen is.
Yemen was a British colony for more than a century and has been a key conduit for East-West trade for hundreds of years. The port of Aden, currently under threat by Yemeni rebel Houthi forces, was where ships used to stop to refuel as they carried goods from the eastern part of the British empire to the U.K.
This strategic importance continues today. More than 3.4 million barrels of oil per day passed through Bab-el-Mandeb – the two-mile strait between Yemen and Africa - in 2013, according to U.S. government statistics.
If that strait was closed, or became unsafe due to attacks from anti-Western forces, oil tankers could be forced to sail all the way around South Africa to reach European, North American, and South American markets.
Saudi Arabia, one of the richest and most influential countries in the Gulf region, borders Yemen, so fighting there represents a threat to its prized stability - particularly given that many of its oilfields are in the south of the country, closer to Yemen. The Saudis also have plenty of money and equipment for a war. One out of every seven U.S. dollars spent on defence imports across the world in 2015 will be spent by Saudi Arabia, according to forecasts by research company IHS.
There are many who forecast that Yemen will be – or already is – the site of a proxy war between Saudi Arabia and Iran, emblematic of the increasingly fractious divide between Sunni and Shiite Muslims.
Yemen has a Sunni majority, but also a significant Shiite minority, with some estimates suggesting the split is around 70:30 Sunni-Shiite. Saudi Arabia, on the other hand, is overwhelmingly Sunni Muslim, but with a significant Shia population near some of its oilfields.
The Sunni-majority Gulf states of the United Arab Emirates, Qatar, Kuwait and Bahrain are behind Saudi Arabia in the coalition of Middle Eastern states which has launched military action in Yemen. Oman, which also has a border with Yemen but a smaller Sunni population, is steering clear of supporting the intervention.
Yemen's Houthi rebels, who are from the Shiite branch of Islam, don't appear to have used the extremist tactics, such as suicide bombings or massacres, of other Sunni militant groups like Islamic State (ISIS) or Al-Qaeda.
Yet they do appear to have the support of Iran, which has raised Saudi concerns further.
The West's nuclear negotiations with Iran are closer to a positive conclusion than they have been for decades, so the U.S. is unlikely to do anything which would throw them off course – such as send in troops to Yemen.
Recently ousted President Abd Rabbuh Mansour Hadi, who was elected in 2012 after the previous ruler was deposed during the Arab Spring uprisings, is in an undisclosed location, believed to be outside the country, and may find it easier not to return to the country if the rebellion is quashed.
Another regime backed by the Saudis, as well as the West, would likely be installed.
CORRECTION: This story has been updated to reflect that many of Saudi Arabia's oil fields are in the south of the country.