THE civil war in Yemen, in which a Saudi-backed coalition has been battling Iranian-supported Houthi rebels, took a new twist this week. On August 2nd the coalition landed at least one armoured brigade at the southern port of Aden. According to reports, the 3,000-strong combined Saudi and UAE force, equipped with French Leclerc main battle tanks, Russian BMD-3 infantry fighting vehicles and American mine-resistant troop carriers, then set off for the Houthi-held military base at al-Anad, some 40 miles (65km) to the north.

Two days later, bombarded from the air and heavily outgunned, the Houthis had fled into the surrounding hills and pro-government forces were back in charge of the strategically important base from which America had until recently flown drone strikes against al-Qaeda in the Arabian Peninsula (AQAP). The fall of al-Anad follows the expulsion of Houthi rebels from Aden two weeks earlier. Al-Anad is the biggest air base in Yemen and, once repaired, can be used by Saudi and UAE transport aircraft and fighter-bombers to widen the scope of the offensive and drive the Houthis out of much of the south.

Despite official claims that the Saudi and UAE forces are only there to help train local anti-Houthi fighters, there is little doubt that the high-tech weaponry is being operated by professionals. They are being helped by tribal fighters who support the internationally-backed government that the Houthis drove out. They know the territory and can hold ground once it has been taken. But it now looks as if the coalition has decided that only well-equipped regular forces and commandos can bring the campaign against the Houthis to a conclusion.

This is both a major escalation and a military gamble. While the Houthi insurgents have nowhere near the quantity or quality of weaponry now being deployed against them, the history of conventional foreign forces fighting in conditions of chaotic irregular warfare is not encouraging. Early gains are all too often followed by military stalemate and quagmire.

Another unknown is how the Houthis’ Iranian backers will respond if their clients are seen to be facing defeat. Iran, whose involvement is anyway limited, can do little to resupply the Houthis, whose airports have been bombed and ports blockaded. Besides, it is already fully stretched in its fight against Islamic State in Iraq and Syria; this is not the moment to turn a small-scale proxy war into an open conflict with the Gulf Arabs.

Even if the Iranians show restraint, the Saudi coalition could get itself into trouble in other ways. It has made some unlikely allies in its effort to crush the Houthis in the form of al-Qaeda and the Muslim Brotherhood, which is treated with suspicion at home. Friends like that also call into question the continued support of the Americans, who consider AQAP the most deadly al-Qaeda franchise, and Egypt’s President, Abdel-Fattah al-Sisi, who regards the Brotherhood as a much greater threat than the Houthis. With the humanitarian situation in Yemen also increasingly desperate, the chances of anyone emerging a winner from the conflict are remote.