The Gulf of Aden is a strategically important shipping route linking the Mediterranean Sea and the Indian Ocean. The port city of Aden controls the Bab al-Mandab strait, the gateway between the Red Sea and Gulf of Aden through which 21,000 ships pass through each year. According to the EIA, 3.8mmbbl/d of crude oil and refined products passed through this route in 2013 alone.
The capture of Aden by the Houthis on 25 March 2015 was therefore a huge concern to the international community, with the US, Saudi Arabian and Egyptian navies stepping up its forces in the region. At the beginning of April, the Chinese were reported to be diverting vessels to the region as well.
At first glance, the involvement of Egypt is unexpected. Egypt had learned not to meddle in foreign affairs after its previous military intervention in Yemen in the 1960s led to the death of 26,000 Egyptian soldiers. Back then, Egypt's President Nasser saw that Yemen was going through what Egypt went through a decade earlier - a revolution against the monarchy, followed by what would be an installation of a republic. Nasser, a champion of pan-Arabism, lent his support to the Yemini republicans. However, what was expected to be a swift war turned into "Egypt's Vietnam" that lasted almost a decade.
One of Egypt's key sources of income is now at risk. The Suez Canal contributed over USD5 billion in tariffs in 2013 to Cairo's coffers, now under threat, means Egypt can no longer turn a blind eye to the developments in Yemen. More importantly for Egypt, its participation in Operation Decisive Storm, allows it to demonstrate its loyalty to the Gulf States which have contributed over USD20 billion of aid in funds and oil products to an ailing Egypt since the ousting of Mubarak in 2011.
However, it would be wise to remember that the Saudis and Egyptians were once on opposite sides of the battlefield in Yemen. The Egyptians had backed the republicans in the 1960s civil way, whereas the Saudis lent their support to the ruling Imam monarchy. Saudi Arabia's policy how not changed though - it and its Gulf allies understandably view the protection of the status quo a priority, i.e. protection from any threat against the ruling monarchies. In the 1960s, it was against Nasser's spread of revolutionary ideology. Since 2011, it has actively worked behind the scenes to undermine the Muslim Brotherhood in Egypt, a moderate religious political group which the Saudi's saw could garner popular support in the Gulf if left unchecked. In 2015, the Iranian backed Houthi movement on the Saudi border is the next, and possibly not the last, challenge to the Gulf ruling system.