Along Saudi Arabia's long, lonely desert border with its northern neighbour Iraq, the Ministry of Interior guards have every reason to stay alert.
Isis, the well-armed and well-funded jihadist army that has seized control of most of western Iraq, is now effectively at the kingdom's doorstep.
Nearly half the 900km (560 mile) Saudi-Iraq border is with Iraq's Anbar province where The Islamic State in Iraq and the Levant (Isis), now rebranded as simply "The Islamic State", is largely able to move its fighters around at will.
Saudi Arabia may not yet be directly in its sights but officials fear this is only a matter of time.
Many of its most violent frontline fighters are believed to be Saudi nationals who may eventually come home, radicalised and brutalised by the conflict.
King Abdullah has ordered "all necessary measures to secure the kingdom from terrorist groups or others who might disturb the security of the homeland".
So how vulnerable to a backlash from Isis is Saudi Arabia, the world's largest oil producer and exporter?
Physically, its northern border is largely secure.
Since 2006 it has been reinforcing a sand berm barrier with an in-depth system of high fences topped with barbed wire, patrolled by access roads, observation posts and cleared strips, backed by military bases.
It is not unbreachable but it does present would-be infiltrators with more of an obstacle than the vague frontier separating northern Iraq from Syria, which Isis bulldozed its way through with such ease last month.
When I visited the Ministry of Interior headquarters in Riyadh last year the ministry's spokesman, Maj Gen Mansour Al-Turki, showed me the models in glass cases of his country's border defences with Iraq.
He seemed relaxed about them compared to the mountainous border with Yemen to the south where border guards have taken several casualties trying to keep out drug smugglers, arms smugglers, economic migrants and infiltrators from al-Qaeda in the Arabian Peninsular (AQAP).
But with al-Qaeda-inspired jihadists now active to both the north and south, Saudi Arabia finds itself somewhat sandwiched in between two dangerous places.
A return to the 2000s?
For the Saudi authorities, the situation bears an uncomfortable similarity to what they were facing a decade ago.
Iraq was in turmoil following the US-led invasion and the dismantling of its central government, droves of young Saudi men were fulfilling what they believed was their religious duty and heading over to Iraq to wage jihad, a holy war.
Today, despite government-approved fatwas forbidding such actions, a number of Saudis have been able to reach Syria and Iraq to join Isis.
Saudi accents have been heard commentating, out of vision, on some of the group's chilling videos of captured Iraqis being executed in cold blood.
For the Saudi authorities, returning jihadists represent a potential domestic security threat if and when they do come back.
That said, many who survive the fighting may choose to either remain in Isis-controlled territory or make their way to Yemen to swell the ranks of AQAP.
So is Saudi behind the Isis success?
"You reap what you sow" seems to be the message coming out of Iran and its allies in the Iraqi government, blaming Saudi for the success of Isis on the battlefield.
"Saudi Arabia's support for the terrorist groups, including Isis, is a plot which will eventually entrap that country too," Iranian parliamentarian Mohammad Asafari said this week.
"Saudi Arabia is the spiritual, material and ideological supporter of the Isis and the Saudi king had tasked the country's former intelligence chief with a special mission to support Isis," he added.
Absolutely untrue, says Saudi Arabia, which in turn blames the Iraqi Prime Minister Nuri Maliki for inadvertently enabling Isis to take over so much territory by marginalising Iraq's Sunnis.
However Saudi officials do not deny that the country's recently retired intelligence chief, Prince Bandar Bin Sultan Al-Saud, has spent an extraordinary large amount of time and money in trying to form an effective Sunni rebel army in Syria to fight the Shia-backed regime of President Bashar al-Assad.
By all accounts his efforts have failed. President Assad remains firmly entrenched in most of the areas that matter and in the fast-moving and dynamic conflict that is Syria the Saudis have not always been able to prevent their material support from falling into the hands of the jihadists of Isis.
So where has Isis been getting its money from?
Not from us, says the Saudi government. But private wealthy individuals in that country are widely believed to have been sending donations through circuitous routes.
Why? Because as Sunni Salafists they resent the creeping influence of Iran and its Shia allies across the region.
For decades Iraq was ruled, brutally, by Sunnis under President Saddam.
The US-led invasion of 2003 changed all that, leading to a Shia-dominated government in Baghdad sympathetic to the Saudis' arch rival, Iran.
So, some Saudis see Isis not as a bloodthirsty bunch of out-of-control terrorists but as a disciplined force safeguarding the rights of the region's Sunnis, a sort of bulwark against Iran and its "heretical" Shia allies.
But a recent US study found that in Isis's surprisingly well-documented records, less than 5% of its funding has come from overseas donations.
Instead, the majority has come from local extortion, kidnapping and unofficial "taxes" imposed on the areas it controls in Syria and Iraq.
Since June it also has access to over $420m (£244m) in cash looted from Mosul's banks.
Isis is now perhaps the world's best-funded and most formidable militia movement and the countries around the region have good reason to worry what it does next.