The capture of Yemen's capital by rebels with ties to Iran has jolted Saudi Arabia, prompting a scramble by Riyadh to prevent its Shiite Muslim rival from exploiting the takeover to make trouble in the kingdom's backyard.
The Sunni Muslim country is also concerned that the security deterioration in its southern neighbor, where the Shiite Houthi fighters seized Sanaa on Sept. 21, does not benefit another old enemy, al-Qaida.
For the hereditary rulers of Saudi Arabia, a stable, wealthy oil kingpin, the 1,400-km (870 mile) border with turbulent, impoverished Yemen which snakes over remote mountains and desert, has always been a security nightmare.