On Saturday, after four days of armed clashes, separatist forces seized control of Aden – the "temporary" capital of Yemen's internationally-recognised government. They appear to have met little resistance in taking over military camps, the home of the interior minister and, most symbolically, the almost-empty presidential palace where guards agreed to leave without a fight.
This might be described as a micro-coup but amid the general turmoil in Yemen it has enormous implications. Besides further complicating the country's multi-dimensional war, it could have far-reaching effects on the eventual outcome.
Firstly, it is a major blow to the already fragile credibility of the government headed by Abd Rabbu Mansour Hadi and to the Saudi-led coalition which supports him as Yemen's "legitimate" president.
Following the resignation of President Saleh in 2012, Hadi – Saleh's deputy – was elected to succeed him in a "contest" where there were no other candidates. This was unconstitutional but it was supposed to be only for a two-year transitional period. Seven years later, though, Hadi is still the theoretical president.
In 2015 Hadi fled the capital, Sanaa, after it was over-run by the Houthi rebels who now control much of northern Yemen, and sought to re-establish his government in the southern city of Aden. In practice though, he has spent most the time since then in Saudi Arabia.
Saturday's events have put paid to Hadi's already tenuous claims to rule from the alternative capital of Aden. As Lady Bracknell might have said, "to lose one capital city may be regarded as a misfortune; to lose two looks like carelessness".
Aside from that, the separatists have now turned long-simmering tensions in the Saudi-led war coalition into an open fissure. The Saudis continue to affirm their support for Hadi while the Emiratis – who in theory also back Hadi but without much enthusiasm – have been giving tangible support to the Southern Transitional Council (STC) which is behind the separatists' takeover in Aden.
Since the start of the war in Yemen the Saudis have focused on bombing the Houthis while the Emiratis have worked with local forces on the ground to fend off the Houthis and try (not very successfully) to stabilise the south. One effect of that was to create a de facto north-south partitition which the southern separatists have been eager to exploit – and the UAE's recent announcement that it is drawing down its forces in Yemen has given them an opportunity to do so.
Following the British withdrawal from Aden in 1967, southern Yemen became an independent state – the People's Democratic Republic of Yemen, under Marxist rule. That continued until 1990 when the south, in the midst of economic and political difficulties, agreed to merge with the northern Yemen Arab Republic. The north, ruled by Saleh, had a much larger population and soon began to dominate what was supposed to be an almost equal partnership.
Four years after unification, the southern leaders had second thoughts and attempted to secede (with backing from Saudi Arabia). They were resoundingly defeated in a brief war and grievances caused mainly by Saleh's subsequent ill-treatment of the south have since led to renewed calls for independence.
Whether independence would be a good idea is debatable to say the least and it has minimal support at an international level. So far, the "southern question" has not been viewed as a priority and the general approach has been to keep it on the sidelines pending a solution of more urgent problems.
Saturday's events look set to change this. If the separatists can maintain their hold on Aden the "southern question" will become impossible to ignore – and that appears to have been the primary objective.
In a letter to the UN Security Council last week the STC made three specific demands:
1. "Credible southern representation" in the political process towards ending the overall conflict.
2. A sustainable political solution for southern Yemen, including a process for self- determination.
3. More international aid for the south.
One problem with placating the southern separatists is that it could make a political solution for the whole of Yemen more difficult. A nationwide settlement would dilute the Houthis' influence by including all the anti-Houthi elements. If the south goes its own way, however, the Houthis' grip on the north is likely to be strengthened – an alarming prospect for the neighbours in Saudi Arabia.