Like a ticking timebomb left unattended for too long, Yemen's undeclared civil war has suddenly exploded into a region-wide crisis that will have far-reaching, unpredictable international consequences, not least for Britain and the US.

The conflict, spreading outwards like a poison cloud from the key southern battleground around Aden, pits Saudi Arabia, the leading Sunni Muslim power, plus what remains of Yemen's government against northern-based Houthi rebels, who are covertly backed by Shia Muslim Iran.

What has until now been an unacknowledged proxy war between Iran and Saudi Arabia, the two great powers of the Middle East, has now burst into an open confrontation that appears to be escalating rapidly as other countries and players are sucked in. The primary Saudi aim is to pacify Yemen, but its wider objective is to send a powerful message to Iran: stop meddling in Arab affairs.