Yemen: Beginning of the end?
Yemen's President Ali Abdullah Saleh is celebrating his 66th birthday on Monday, confronting the reality that a powerful military rival - acting in the name of Yemen's popular revolution - is making a bid to remove his family from power.
Tanks under the command of Mr Saleh's eldest son, Ahmed, are parked at the gates of the palace in the capital, Sanaa. Tanks are also stationed outside the ministry of defence and the central bank.
These defensive deployments to protect Mr Saleh's control follow the announcement that military commander Gen Ali Mohsin al-Ahmar has voiced support for Yemen's pro-democracy protesters.
The general's defection follows similar moves by a growing number of ministers, ambassadors, parliamentarians and prominent businessmen during the past few days.
Defections have gathered rapid pace since Friday, when snipers opened fire on a pro-democracy camp in the capital, Sanaa, killing more than 50 people.
US officials condemned Friday's violence "in the strongest terms" and expressed hope that Yemen might still achieve a political solution through negotiations and dialogue.
The US administration has been pushing the same line on negotiations and dialogue ever since Yemen's pro-democracy movement began to gain momentum in January.
However, the trust required to achieve a political settlement has been absent and Yemen's opposition politicians calculated that they had nothing to gain by cutting a power-sharing deal.
As a result, stalemate prevailed, while young Yemenis - frustrated with the failures of an entire generation of politicians - took to the street in ever greater numbers.
The US administration's recent attempts to encourage bipartisan dialogue have been based on a fundamental misreading of the real political dynamics in Yemen.
While diplomats have focused their attention on the established political parties, the gaps between the formal institutions of the state and the elaborate networks of patronage and corruption that are closely entwined with Mr Saleh's regime have gradually been moving into open view.
With Gen Ali Mohsen al-Ahmar's defection, long-standing competition between different factions within the regime has finally been exposed.
Now, Yemenis are waiting to see what happens next, and Twitter is buzzing with speculation. Many Yemenis are expressing jubilation, or stunned disbelief, at the prospect that Mr Saleh might be removed from office after more than 30 years in power. Others are warning of a massacre - or civil war.
Pro-democracy protesters are nervous that their popular revolution will be hijacked by established military and commercial interests, who will simply nominate a new face to govern the country without making any substantial changes to the status quo.
In this scenario, Gen Ahmar is likely to act as a kingmaker, while opposition politician Hameed al-Ahmar may emerge as one of the beneficiaries.
US officials may now have to make a rapid recalculation about their short-term and long-term interests in Yemen.
To date, they have taken a cautious approach with Mr Saleh - one that seemed out of step with much bolder messages delivered to Egypt's Hosni Mubarak and Libya's Muammar Gaddafi, who were both told that the game was up within a matter of days of popular unrest in their respective countries reaching a critical tipping point.
A crucial factor influencing US decision-makers is their assessment that al-Qaeda in the Arabian Peninsula, based in Yemen, ranks among the most active branches of the global terror organisation.
For several years, the US administration has been supplying military aid and training to elite security and intelligence units under the command of Mr Saleh's son and nephews. The White House is nervous about losing these relationships with local proxies, who have been willing to co-operate in US counter-terrorism operations.
However, the longer US officials try to keep Mr Saleh and his family in place, the more they risk damaging their own interests. Yemenis are furious that units from the US-backed Central Security Forces, commanded by one of Mr Saleh's nephews, have played such a prominent role in cracking down on pro-democracy protests.
US-made CS gas canisters, allegedly intended for counter-terrorism operations, have also been used in raids against pro-democracy protesters.
For many years, Mr Saleh has adeptly played on fears of chaos if he loses control in Yemen but his internal support is swiftly ebbing away.
Ginny Hill runs the Yemen Forum at Chatham House, an independent international affairs think tank.