Following the Yemeni unity in 1990, and particularly after Kuwait liberation in 1991, Yemen and Iran recovered their normal diplomatic relations after a rupture of relations with the north and semi alliance with the south, however Iran stood on the side of unity during the 1994 war, refusing the separation project, that was adopted by its former ally Ali Salem al-Bydh.
There was no real cooperation with Ali Abdullah Saleh's regime who won the war, however the relations between the two countries made no progress despite the formation of a joint ministerial committee between the two countries to strengthen their bilateral relations, especially Iran had expressed willingness to provide developmental support to Yemen after the 94' war.
I remember attending a meeting with the then Parliament Speaker in 1997, late Sheikh Abdullah Bin Hussein al-Ahmar, when he received the  Iranian minister Jihad al-Bina, who visited Sana'a to head his country's side in the joint committee.
I was surprised at the meeting with the severe criticism of Sheikh al-Ahmar to the Iranian minister, telling him his government is not serious in supporting Yemen, and all it provides are false promises. Sheikh al-Ahmar said such method is not acceptable, stressing that either Iran proves its seriousness in the developmental support to Yemen or there is no need for the annual meetings of the joint committee.
Sheikh al-Ahmar was keen to send a clear serious message that the Yemeni government failed to convey to the Iranians, who didn't change their technique in their relations with Yemen since then, although former President Saleh went in an official visit to Iran in 2000, and an official visit was also held by the former Iranian President Mohammed Khatami to Yemen in 2003.
There was no positive consequences to the visits, however the Iranians had worked quietly to serve their political interests through modest medical centers established in Yemen, in addition to the opening of commercial facilities to Yemeni figures sympathizing with Tehran, intellectually, politically and in doctrine.
When the military confrontation escalated between Saleh's regime and the Houthis from 2004-2010, these centers and others became the eyes that were observing closely what is happening, whereas the commercial guise was an ideal method for financing the forces sympathizing with them.
Iran knows that the countries that undergo transitional periods are exposed to various kinds of vacuum which continue until the political stability is recovered.
Such periods which are dominated with political confusion, countries and forces try to impose their visions in configuration of the future of the country at such transitional periods.
This is exactly what Iran is doing in Yemen, in a position entirely contradicting the political consensus, reached by the political forces under regional and international sponsorship, boosted by two resolutions issued by the Security Council.
With the exception of the strong blatant sympathy with the Houthi movement during the fifth and sixth war, this sympathy has to some extend retreated, even during the beginning of the youth peaceful revolution at the beginning of last year. The Iranian interest came back strongly with the emergence of signs of collapse of president Saleh an al-Asaad's regimes.
Iran didn't expect President Saleh to be serious in power transfer, and its concern used to increase with its feeling to weakness of Saleh's fading  excuses for not signing the GCC initiative, especially this had coinciding with the popular revolution in Syria. Tehran knows the inevitability of the fall of its Syrian ally, due to its exaggerated massacres and the spread of the revolution against him.
Like any major power, having huge regional interests, it has to think of new serious techniques for finding new footholds to replace its collapsing influence.
If Yemen represents a potential Iranian alternative to Syria, the ayatollah in Tehran cannot ignore the differences between Syria and Yemen, because the Syrian Iranian alliance, was established with the consent of the sectarian regime in Damascus, 40 years ago. This alliance wouldn't have continued, had it not been to the continuation of the Asaad's regime. However this wouldn't have been possible in Yemen and cannot be possible in future.
The fall of the former Iraqi president Sadam on the hands of the Americans and their forced or voluntary handing of power to a regime in Baghdad  closer to the Iranian mullahs in Tehran, created for the first time in history a geographical continuation to Iranian regime up to the Mediterranean , extending across Iraq, Syria and Lebanon through their intimate relation with Hezbollah, which represents a difficult figure in the current Lebanese government.
The fall of Asaad's regime will end such continuation and affects all Iranian cards in the Arab region. This explains their thinking of Yemen, but in a policy though not totally new, is developed to expand its perspectives and dimensions.
It was not easy for President Hadi to set off the bomb of the Iranian intelligence activity in Yemen. Though he was known for his little speeches but he launched unprecedented warnings, followed by the disclosure of the Iranian espionage cell, working for a long time in Yemen, directing many activities, escalating lately to form serious threats against its security, stability and national unity.
President Hadi and his government's concern came after the long silence towards the Iranian subversive activity, eminently continuing in Yemen.
Iran is not contented with its old known strong support to the Houthis, but it expanded its activities since late last year in a rapid manner of sectarian and regional conflicts' instigation. 
It is openly supporting Yemeni separatist forces in the south, in addition to its attempts for stirring up Sunni sectarian conflicts in the north aiming to tear up the doctrine of Yemeni north.
This is a satanic idea of the traditional Iranian allies in Yemen who enjoy its support to their racist and sectarian doctrines.
I think the Iranian generous support to the two latter parties had not given its results yet and one believe it won't, not because of the early awareness in confronting it, but most importantly for its contradiction to the Yemeni desire, expressed through the peaceful revolution.
 The involvement of Iran, which is a country ruled by a number of theocratic reactionary religious men not representing any developmental method that can be followed, like Turkey and Malaysia also forms an alienating element. It is principally not liked by most of the Yemeni people who are skeptical over all its subversive projects in Yemen.