The Houthis are seen to be representing Yemen's Shia minority in their fight against Sunni groups. Photo / AP
The CIA's drone base in the rippled surface of the Saudi Arabian desert has been expanded over the past few years. Satellite imagery shows dozens of additions that appear to include living quarters, a new clamshell hangar for hiding aircraft and neat rows of freshly planted palm trees.
That base is one of the few components of the Obama Administration's counterterrorism campaign in Yemen that remain intact.
Agency and United States military personnel have been pulled out of Yemen amid escalating sectarian violence in recent weeks. Elite Yemeni units that the US trained to hunt al-Qaeda have been scrambled by the Government's collapse. And millions of dollars in US-provided military equipment has been destroyed in a span of days by Saudi airstrikes aimed at rendering those arms useless to the Iran-backed rebels who control the capital.
The vacuum, US officials say, appears to have allowed al-Qaeda in the Arabian Peninsula (AQAP) to focus on rebuilding its strength after years of US strikes against its leaders.
A prison break in eastern Yemen on Friday freed as many as 300 inmates - including a senior AQAP leader - in an operation seen as part of a broader effort by the group to shore up its ranks.
US officials said the CIA's armed drones are still flying over Yemen, prepared to launch strikes against AQAP members.
Officials also insisted that US intelligence support to the Saudi air campaign has not diverted resources from tracking the group.
But the counterterrorism fight has gone from the most active battlefront in Yemen to a secondary conflict, swallowed up by a civil war that is serving as a proxy for a broader regional struggle between Saudi Arabia and Iran.
The US has not made a drone strike in Yemen since mid-February, when Houthi rebels formally declared their takeover of the Government. The drone campaign has been characterised by such pauses for several years, but US officials said that they are likely to become more common and lengthy as ground-level intelligence missions in the country end.
"With the deterioration in security, and a diminution in counterterrorism co-operation, the pressure has been taken off AQAP," said Representative Adam B. Schiff of California, the senior Democrat on the House Intelligence Committee.
"We still obviously have intelligence in Yemen," he said, "but we're more reliant on our overhead assets."
The chaos would appear to give AQAP a chance to ramp up terrorist plotting against the West while also asserting itself as the defender of Sunni Muslims across Yemen who are threatened by advancing Shia-dominated Houthi militias.
Before the Friday prison raid, though, AQAP had been relatively inactive.
Aside from claiming credit for a series of small-scale attacks against Houthi fighters - who see Sunni-dominated al-Qaeda as an adversary - the group has avoided exposure to more direct confrontations or lingering American drones.
"The initial evidence is actually that the Houthi advance has caused [AQAP's] external plotting to be sidelined while they figure out how they're going to deal with ... what appears to be an emerging civil war," said a senior US military official, who like others spoke on the condition of anonymity because of the sensitivity of the matter.
For now, the US and al-Qaeda are in oddly similar positions of warily assessing the course of the unfolding war in Yemen and the impact of that conflict on their abilities to proceed.
"The [US] counterterrorism strategy has been kicked aside for a while," said Khaled Fattah, a Yemen expert, adding that he expects AQAP to become increasingly involved in fighting the Houthis, especially in the country's southern and eastern provinces. US officials continue to see AQAP as posing the most direct danger to the US, even amid the rise of new terrorist groups including Isis (Islamic State). Al-Qaeda's Yemen franchise was linked to the attacks in Paris in January as well as to previous attempts to detonate bombs on US-bound aircraft.
The CIA's airstrip in Saudi Arabia has been a critical hub in the US assault on AQAP, serving as a base for remotely piloted aircraft that have made dozens of strikes, including one that killed the US-born al-Qaeda operative Anwar al-Awlaki in 2011.
Satellite images show that the remote facility has expanded since then, underscoring its importance to a campaign whose reliance on US technical prowess has defined President Barack Obama's counterterrorism approach.
But other aspects of that strategy - including the US dependence on local security forces to shoulder on-the-ground risk - have unravelled since its most staunch supporter in Yemen, former president Abed Rabbo Mansour Hadi, was forced from office and then fled the country.
Administration officials have sought to play down the damage to US capabilities. White House press secretary Josh Earnest said recently that while instability in Yemen "does not enhance our counterterrorism efforts ... we continue to have significant counterterrorism resources and abilities".
Current and former US officials said the American ability to find drone targets hinged on other streams of intelligence that have been disrupted, if not severed, amid Yemen's downward spiral. Among the most critical sources of intelligence for the airstrikes was a network of informants established and run by Saudi Arabia, whose security services are presumably preoccupied with finding Houthi targets for the kingdom's fighter jets.
"The human contact that allows intelligence to be exquisitely defined is now lacking," the senior US military official said.
"Our other forms of intelligence remain available to us - it's just a diminished capacity."
In a measure of how chaos has confused battle lines, Saudi Arabia and AQAP are now focused on a common enemy in the Houthi rebels.
AQAP has not condemned Saudi Arabia's intervention in the conflict, according to Yemen experts who see the group's silence as an indication that its leaders believe they stand to gain from the Saudi strikes.
The prison break on Friday was both a brazen move by AQAP to replenish its ranks and a sign of how completely security in the country has collapsed.
Counterterrorism experts said that AQAP may feel pressure to consolidate its position in Yemen before asserting itself militarily, but is unlikely to abandon its commitment to plots against the US.
Shia-Sunni conflict
•The Houthis are a Shia insurgency group who originated from northwestern Yemen's Saada province. Their origins lie in the Shabaab al-Mumanin (the Believing Youth), a group that operated in the early 1990s.
•They became powerful as a result of a long period of armed conflict with the Government that turned the Houthis from student activists to seasoned insurgents.
•The Houthis are Shia, and many of the forces they are fighting are Sunni. Part of their appeal comes from the idea they are representing Yemen's Shia minority, which is estimated to account for 35 per cent of the country's Muslims.