For example, the successful Shia Houthi rebellion has been backed not only by Iran, but also by parts of the armed forces loyal to former Yemeni president Ali Abdullah Saleh, who is Zaidi Shia. It is also another manifestation of Saudi Arabia's regional competition with Iran. Needless to say, Shia nation-states Syria, Iraq and Iran are all opposed to any Saudi involvement in Yemen.

Who are the Houthi and why are they fighting?

The Houthi is a Zaidi Shia movement from north-west Yemen. It takes its name from Hussein Badreddin al-Houthi, head of the Zaidi Shia sect, who launched an insurgency in June 2004. He was killed by government forces in September 2004.

The Yemeni government claimed that the Houthi were seeking to overthrow it and implement Shia religious law. The rebels countered that they were simply defending themselves against discrimination and government aggression. The government also accused Iran of directing and financing the insurgency.

In August 2009, the Yemen Army launched a major offensive against the Houthi. Hundreds of thousands of people were displaced by the fighting. The rebels accused Saudi Arabia of supporting the Yemen government against them and clashes broke out between the rebels and Saudi security forces along the two countries' common border. The Houthi also claimed that in December 2009 the US launched 28 air strikes against them.

The government offensive lost momentum after General Ali Mohsen al-Ahmar, who had led the operations from 2004 until 2011, resigned to defend protesters during the Yemen Arab Spring Revolution. Until 2011 most of the fighting had taken place in Sa'dah Governorate in north-west Yemen and neighbouring governorates.

President Ali Abdullah Saleh had been the autocratic ruler of Yemen since 1990 until he was forced to step down in February 2012 as a result of the increasingly violent street protests. When he resigned, his Sunni deputy, Abed Rabbo Mansour Hadi, became president.

On January 20, 2015, Houthi rebels surrounded the presidential palace in the capital Sana'a, forcing Hadi to resign. Presidential guards surrendered after being assured that Hadi could leave safely. The following month he made his way to the southern port city of Aden where he rescinded his resignation.

In the past month Houthi forces have advanced south towards Aden, following violent attacks in the capital Sana'a that they said were being organised from the south. When the Houthi were closing in on Aden, Hadi fled to Saudi Arabia.

Saudi Arabia, supported by the Arab League, has established a Saudi-led coalition of 10 military forces dubbed Operation Decisive Storm to defeat the Houthi and restore President Hadi to power.

Saudi Arabia's confidence to lead this operation may have been buoyed by its military success in 2011, when it intervened in Bahrain in support of the Sunni monarch to put down an insurrection by the Shia majority. That was a far less complex undertaking, against mostly unarmed protesters.

Developments in Yemen have complicated US Middle East policy. In Iraq the US is supporting a Shia government against the advance of Sunni extremist group, Islamic State (IS). This has put it on the same side as Iran which seems to be doing most of the ground fighting for Baghdad, while the US is carrying out most of the air strikes. Meanwhile Saudi Arabia has reportedly been providing backdoor support to IS through Syria.

By contrast, in Yemen, the US is supporting Saudi Arabia against Shia Houthi rebels who are fighting against the powerful Yemeni affiliate of al-Qaeda (AQ), AQ in the Arabian Peninsula (AQAP). The Houthi are also trying to contain IS which is seeking to gain a foothold in Yemen. The Houthi would therefore seem to be on the same side as Washington as long as they do not attack Saudi Arabia. Paradoxically US operational support is helping AQAP and IS.

Egyptian officials have said that Saudi Arabia and Egypt will lead ground operations in Yemen against the Houthi militants once they have been sufficiently weakened by Saudi-led airstrikes. According to the officials, the ground forces will enter by land through Saudi Arabia and by sea from the Red Sea and the Arabian Sea.

So far the air strikes seem to have mainly killed civilians, with little impact on the fighting capabilities of the defiant Houthi and their allies.

Washington has announced it will provide logistical and intelligence support to the operation. With this intelligence support, Saudi-led forces might be able to conduct precision strikes but as we know from long experience, air strikes are not going to win the war unless they are supported by effective ground operations.

Although numerically the Saudis, Egyptians and other Arab countries should easily prevail over the Houthi - that is not a likely outcome given relative military capabilities.

The Saudis say they will be deploying 100 jets and 150,000 troops, but neither the Saudis nor Egyptians are particularly competent militarily, while the 100,000 strong Houthi are battle-hardened fighters operating on home ground.

Saudi Arabia has excellent military equipment but is not very experienced at using it. During the Gulf wars against Iraq in 1991 and 2003 the Saudis sat on the sidelines and let the US-led coalitions do the heavy lifting. Its new best ally Egypt has fought several wars against Israel, and invariably been trounced by a smaller defence force.

The Houthi are now led by Abdul-Malik Houthi, born 1982, brother of the late Hussein Badreddin al-Houthi. His brothers Yahia Badreddin al-Houthi and Abdul-Karim Badreddin Al-Houthi are also leaders of the movement. While the Houthi movement can probably continue to dominate the western side of Yemen, they lack the capability to conduct operations into east Yemen.

Clive Williams is an adjunct professor at Macquarie University's Centre for Policing, Intelligence and Counter Terrorism and a visiting professor at the ANU's Centre for Military and Security Law.