There is a sign on French level crossings for those who consider themselves to be vigilant. It reads "One train can hide another". This is sound advice should be erected in Oval Office and Downing Street. Only it should read: "One intervention can hide another."

While all eyes are on Syria and Iraq, another intervention is taking place in Yemen. The coalition involved here is more of an understanding than a military pact, but it is proving to be just as effective.

That Iran should be backing a small Shi'a tribe in the north of Yemen called the Houthis comes as little surprise. The lightning takeover by the Houthis of the capital Sana'a has been applauded by Iranian politicians. Tehran city representative in the Iranian parliament, Alireza Zakani, a loyalist of the Supreme Leader Ali Khamenei, boasted that Iran now controls four Arab capitals - Baghdad, Damascus, Beirut and now Sana'a.

"Three Arab capitals have today ended in Iran's hands and belong to the Islamic Iranian revolution". He noted that Sanaa has now become the fourth Arab capital that is on its way to joining the Iranian revolution.

Zakani said that Iran considered the Yemeni revolution to be an extension of its own and that 14 out of 20 provinces would soon come under the control of the Houthis and that they would not stop there:

"Definitely, the Yemeni revolution will be not be confined to Yemen alone. It will extend following its success into Saudi territories. The Yemeni-Saudi vast borders will help expedite its reach into the depth of Saudi land."

So far, this is business as usual in Yemen. The surprise is to hear how closely involved Saudi Arabia and its ally the United Arab Emirates were in the Houthi advance and how it reached a de facto understanding with their biggest regional rival, Iran. In November last year I first reported Saudi contacts with their old enemies the Houthis. I wrote then:

"Bandar's war against political Islam has also made itself felt on Saudi's troubled border with Yemen. The need to combat the advance of Islamist group al-Islah in Yemen has led the Saudis to support Houthi militias - with whom the kingdom once went to war. A prominent Houthi, Saleh Habreh, was flown via London to meet the Saudi intelligence chief."

In the last few weeks those meetings have intensified. I understand that a Houthi delegation went to the UAE for a high-ranking meeting and that the same delegation flew on to Riyadh. There has also been a successful meeting between Iranian and Saudi Arabian foreign ministers in New York a week ago.

The point man for these meetings was the Yemeni ambassador to the UAE, Ahmed ali abdullah Saleh, the son of the former president who was forced from office in 2012.

After his meeting with the Saudi foreign minister Prince Saud al-Faisal, his Iranian counterpart Mohammad Javad Zarif, suggested the talks could lead to an improvement in relations.

"Both my Saudi counterpart and I believe that this meeting will be the first page of a new chapter in our two countries' relations," Iran's official IRNA news agency quoted Zarif as saying.

The Houthis and Iran are staunch enemies of al Qaida in the Arabian Peninsula, the most active branch of the franchise, outside Syria and Iraq. But the target of this sudden advance is al-Islah, the reformist Islamist party. In this, the Houthis appear to have been helped by the former president Saleh, who still remains a key powerbroker. Saleh, from retirement, has made statements hinting at support for the Houthis. When they arrived in Sana'a he replaced his profile photograph on his FaceBook page with one of him smiling.

Yemen's history has been littered with febrile military alliances, which fracture at the slightest opportunity. Almost everyone has fought everyone else at some stage. The sectarian element is less pronounced in Yemen than it is in elsewhere in the Gulf because the split between Sunni and Zaydi tribes is much less than it is between Sunni and other Shi'a groups.

The Zaydis are often referred to as the most moderate of the Shi'a groups and closest to the Sunnis in their theology. The Zaydis do not consider Ali ibn abi-Talib and his descendants as divine as other Shi'a groups do. They simply preferred his rule. It is a political preference rather than a theological injunction.

In today's conflict many of the leaders of al-Islah are themselves Zaydi. They are from the Hashed tribe as is Saleh himself. So to call this conflict sectarian is misleading.This is a political conflict in which the government forces, which fought alongside al-Islah against the Houthis, have melted away or sided temporarily with the small northern tribe from Sadah in North Yemen.

Islah in Yemen have never been in conflict with the Saudis , but Riyadh appears to have made a strategic calculation that now is the time to declare war on all political Islamist factions wherever they happen to be in the Arab world - Yemen, Egypt, Libya.

When asked in a meeting in Europe recently why the Saudis were so staunch in their opposition to moderate political Islamists, Turki al Faisal, the foreign minister's brother, and former ambassador to Britain said they had made a strategic calculation. He said:" This policy has been thoroughly thought out."

It may have been thought through, but it is a dangerously short-sighted calculation particularly when applied to a country like Yemen which straddles Saudi Arabia's porous southern borders. It is a high risk strategy on two fronts. Where will Sunni allegiances lie if Islah is destroyed? A major political Sunni grievance in both Syria and Iraq created the Islamic State, and the same process is now underway in Yemen. Just recall what the Saudis did to crush the Arab Spring in Bahrain, claiming that Iran was at its gates. Today they are using Iranian backed forces against Sunni Islamists in Yemen. Anything goes, in any Arab country, as long as the target is political Islam.

But it is also perilous for the Saudis to assume that Iranian influence is a one way street. The Iranians calculate that after the western-led coalition crushes the Islamic State in Iraq and Syria, its client Bashar al Assad will be next. The possession of Sana'a is an important card Iran can now play if Assad is threatened. Zakani's prediction could always come true. If Saudi Arabia is part of a coalition which then turns on Iran's chief ally in Syria, the Iranians could threaten the kingdom on its southern front.

Some analysts claim Britain and America are up to its neck in this intrigue. The Yememi analyst Yassin al-Tamimi wrote:

"What happened in Sanaa had already been planned for by the Americans, the British and the Saudis and agreed upon by these parties. The evidence is that the Friends of Yemen meeting which was held last Wednesday in New York did not condemns the Houthis' armed invasion of the capital. This points to international collusion with what the Houthis militias did."

The Houthis, whose armed campaign has previously been condemned by the UN Security Council well before the current campaign, have stormed and looted houses in Sana'a. They stormed the house of the Noble Peace Laureate and Yemeni journalist Tawakkol Karman, calling her a Takfiri. Happily she escaped.

The more pragmatic princes in Saudi Arabia's ruling family should think about what will happen next. What are the consequences of failure? What if these "thoroughly thought out" tactics were used against them on home turf?