How would ISIS spread into Saudi Arabia?
More generally, how would an open source insurgency defeat a fragile dictatorship?
As I pointed out earlier, ISIS is already doing a great job marketing itself to young male populations (particularly young men in Saudi Arabia). That's to be expected. Open source insurgencies are very adept (many hands method) at finding the elements of the message that resonate (throw against the wall method), and then amplifying them a million fold via social media. We've seen this in Tunisia, Egypt, Libya and Hong Kong.
However, to tip the scale in favor of a fast collapse in Saudi Arabia, it's going to take a bit more than viral media. So, what are we likely to see?
Based on what we've seen open source insurgencies do in the recent past, we're likely to see a physical disruption of the infrastructure that supports Saudi Arabia's heavily urbanized (read: urban = fragile) population. This is likely to be very effective. Saudi cities have grown far beyond any rational limits, particularly given the constraints of their locations. This makes them extremely vulnerable to systems disruption (systemsD for short).
Jeddah (pic above) is a good example. Saudi cities like Jeddah are large and extremely dependent on the smooth functioning of critical infrastructure.
This extreme dependency makes them extremely good candidates for disruption by small groups of self-activating jihadis. In Jeddah's case, a couple of dozen small attacks on the water (in/out) and the electricity grid would almost certainly shut the city down. Further, as we've seen on numerous occasions (from Nigeria to Iraq), a small number of attacks is all that is necessary to keep the infrastructure in perpetual malfunction.
This type of disruptive attack, only requires a few individuals acting on their own to be successful. It doesn't require much planning and if these attacks are done correctly, there's only a small chance the attacker will be caught.
Despite that minimal requirement, the impact on battlefield would be extreme. How so? As we saw in Iraq, critical infrastructure will completely gut a city's economy by making anything but barter impossible to accomplish.
Further, these attacks won't create outrage against the attackers. On the contrary, this disruption (particularly if it is bloodless) will cause the population to resent the government since it's proving that it is unfit to rule the country by not delivering electricity, water, fuel, etc. services more than a couple of hours a day. Further, shortages will force much of the population to reaffirm their primary loyalties to the detriment of the national government.
The final benefit of this disruption, particularly if it is made deep inside Saudi territory, is that it will cause the royals to spread their forces thin in a useless attempt to prevent future urban disruption.
This means that when ISIS begins to make cross border raids, the Sauds will be far less likely to defeat them.
NOTE: Deep disruption is, incidentally, the same strategy that Lawrence of Arabia used against the Turks.
PS: Here's some basic info on systems disruption.