The official aim of the Saudi-led military intervention in Yemen is to restore the "legitimate" government of the internationally-recognised but unconstitutionally-elected President Hadi. That was always something of a fantasy and events during the last couple of months have made it even more unlikely.

After a brief period in the 1990s when Yemen assumed the appearance of an emerging democracy, the country is now on a fast rewind, politically, to what it was sixty years ago. The north is once again under Zaidi rule and the south, formerly a British protectorate, is rapidly turning into an Emirati protectorate.

The war itself has caused a monstrous humanitarian catastrophe and, militarily, it's at an impasse. The first serious attempt to break the deadlock came in December when Ali Abdullah Saleh, the disgruntled ex-president, attempted to stage a coup in the capital, Sana'a.

Saleh, who had previously held power in Sana'a for almost 34 years, resigned under international pressure in 2012 following a popular uprising against him. His deputy, Abd Rabbu Mansour Hadi, then became "temporary" president through a one-candidate "election" even though the constitution said there must be at least two candidates.

As a reward for resigning, Saleh was given immunity from prosecution and allowed to remain in Yemen – which gave him plenty of opportunities for plotting against Hadi.

Saleh also retained influence among the military and in 2014 he helped the Houthis – Zaidi rebels from the far north against whom he had previously fought six wars – to seize the capital, giving them control over the northern part of the country. Hadi, meanwhile, fled south to establish a temporary capital in Aden, though because of the security situation there he seems to have spent most of his time in Saudi Arabia.

In a dramatic move at the beginning of December, Saleh suddenly turned against his Houthi allies and tried to stage a coup in Sana'a. The attempt failed and Saleh was assassinated.

Had the coup succeeded it would have changed the course of the conflict, seriously weakening the Houthis (who would probably have had to retreat to their mountain strongholds) but re-establishing Saleh as a key political figure.

The failure of Saleh's coup was also a setback for the Saudis and Emiratis who have been waging war on the Houthis for almost three years. Success might have hastened the end of the war and raised hopes for rebuilding Yemen as a single country under a single government, though Saleh would undoubtedly have exacted a price for his contribution.

On the non-military front, this leaves the Saudis and Emiratis with a couple of serious dilemmas. One is the Hadi problem. The other is the question of partition.

The Saudi-Emirati claim to be defending "legitimacy" in Yemen is based around Hadi's presidency and it's wearing thinner and thinner. The way he became president was undemocratic though at the time it was widely accepted by Yemenis on the grounds of expediency and on the understanding that his term would last only two years. By the end of next month it will have lasted six years.

Hadi is also an uninspiring figure with no real power base in Yemen and not much of a popular following. Even in the south – his "temporary" base – his power is contested. The Saudis continue to support him but the Emiratis would probably be happy to ditch him. In a sense, though, Hadi is irreplaceable because under present conditions there is no possibility of holding a fresh country-wide presidential election.

This brings us to the second of the Saudi/Emirati dilemmas and the second recent development: the challenge from southern separatists.

Following the British withdrawal from Aden in 1967 the south became the People's Democratic Republic of Yemen, under Marxist rule. The north, meanwhile, was emerging as a republic after the toppling of the Zaidi imamate and a civil war in which Saudi Arabia supported the royalists and Nasser's Egypt backed the republicans.

The north-south division into two states was largely artificial – the result of British and Turkish imperialism. Although there are cultural differences among the various parts of Yemen there was also a sense of shared Yemeni identity and both sides had aspirations towards national unity. Talks about unification took place, off and on, for more than 20 years, interspersed with periods of conflict.

The two states finally merged in 1990, largely because the south was in dire straits economically and its Marxist leaders were worried about the collapse of communism in eastern Europe.

Unification began as an almost equal partnership between the former regimes of north and south, but that didn't last long. As Saleh began to assert himself, the south had second thoughts and in 1994 fought a war of secession which Saleh's forces won in the space of a few weeks with support from Islamist elements.

Saudi Arabia, which had opposed unification from the start, supported and armed the southern separatists and later gave refuge to some of their leaders. Since then, though, the kingdom has come full circle and its official position now is to support a single Yemeni state with Hadi as its president. Events, however, are pushing in a different direction.

For all practical purposes Yemen is, once again, two countries. This has happened more by accident than design and is largely a reflection of current battle lines, plus the way the military alliance carved up responsibilities with the Saudis focusing on aerial bombing of the Houthi-controlled north and the Emiratis, more active on the ground, trying to secure the non-Houthi south.

While the de facto partition might be viewed as temporary there are growing signs that it could become permanent. Exasperated with Hadi, the Emiratis now seem to regard the southern separatists as a better bet.

On Sunday, fighting broke out in Aden between Emirati-backed separatists and forces loyal to Hadi. According to latest reports it's still going on and at least 21 people are dead. It's not the first time these two factions have come to blows but the current outbreak is the most serious so far.

The likely effect of this struggle will be to further consolidate the north-south divide but without the independence that the separatists seek. The south doesn't have the capacity or the resources to become truly independent any time soon – there are simply too many problems, not least of them security. Its only choice, basically, is to be ruled from Sana'a or from Abu Dhabi.