What motivates Saudi Arabia to fight Yemeni rebels?

Caryle Murphy delves into the war between Saudi Arabia and the Yemeni ebels, trying to find out what is behind it:

Saudi Arabia’s ongoing military offensive against rebels in neighboring Yemen — the first time its armed forces have gone into combat in almost 20 years — underscores Riyadh’s deep concern about Yemen’s crumbling internal stability, and the possibility that Iran will exploit the turmoil to spread its influence.
The Saudis’ sustained air-and-ground offensive against the rebels, known as Houthis, is also raising questions about Saudi objectives and how they will extricate their forces from what is a messy and volatile internal struggle in an increasingly dysfunctional state.
“It is hard to know what the Saudis intend to achieve in terms of specific military objectives,” Kristian Ulrichsen, Kuwait Research Fellow at the London School of Economics, wrote in an e-mail. They “would be well-advised” not to get involved in what “is rapidly becoming a failed state” in Yemen.
Yemen’s slide into ungovernability has raised alarms among counterterrorism officials everywhere because of the presence there of a potent Al Qaeda franchise. The fears are particularly acute in Saudi Arabia because Al Qaeda in the Arabian Peninsula (AQAP), as the group calls itself, is infiltrating operatives into the kingdom from Yemen.

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