There is no good guy and no bad guy in the conflict between Iran and Saudi Arabia (and its GCC allies). In its essentials, it is a struggle for regional dominance between two benighted and malign powers.
The theater of the conflict is Iraq and Syria. Iran has some advantages, most notably, it is allied with the government of Syria (now supported by Russia), and for sectarian and geographical reasons has advantages in Iraq. In Syria, Saudi Arabia and its allies must resort to funding and arming the opposition. Its options in Iraq are more limited, but it is likely objectively pro-ISIS.
Neither Iran nor Saudi Arabia can pose a conventional military threat to the other. Iran’s air force is a collection of museum pieces (F-4s, F-14s and F-5s!) seized from the Shah and kept together with bubble gum an duct tape, and some Russian aircraft gifted to them by a desperate Saddam 25 years ago. Iran’s ground forces have no power projection capability. Its units have struggled in Syria and Iraq, and were noted during the Iran-Iraq war mainly for their ability to absorb appalling casualties. Iran’s navy also lacks any power projection capability. Logistics would also render impossible any Iranian attack on KSA.
Saudi Arabia has a very well-equipped air force, with 70 F-15E strike aircraft, 86 F-15C and D air superiority fighters, 72 Eurofighter Typhoon multirole aircraft, and 80 Tornado ground attack planes. This is more than adequate to defend KSA against anything that Iran could throw against it on air, sea, and land. But KSA’s ground forces are, like most Arab armies, woefully ineffective, and mainly intended for regime protection. The Saudis are bogged down in neighboring Yemen, and could not hope to project any force into Iraq, let alone Iran.
Even if Iran develops a nuclear weapon, it will have little effect on the balance of power. The Saudis will almost certainly obtain one as well, and a nuclear weapon is more of a regime protection weapon than an instrument of power projection.
Iran’s main weapon is subversion, but this is difficult to employ against a police state like KSA. Indeed, the execution of Nimr Nimr that precipitated the latest crisis was no doubt a signal to Iran that the Saudis were willing to use extreme measures to crush any uprising in the Shia population in the eastern provinces. Also look at the brutal crackdown in Bahrain to see how the KSA and its allies deal with Iran-fomented Shia internal dissent.
So there will be an intensified shadow war between KSA and Iran, fought mainly in Syria and Iraq. Things have intensified now because the Iran deal and the Russian intervention in Syria disturbed the previous equilibrium in the region: this was one of the main reasons the Iran deal, and the administration’s subsequently fecklessness in responding to Iranian provocations, was so ill-advised. The most likely outcome is an intensified struggle resulting in a renewed stalemate.
In terms of oil, the most likely outcome is that the Saudis will figure that Iran suffers from lower oil prices more than they do, so they will not cut output. The Iranians have every incentive to produce as much as they can.
There are loud calls from some quarters that we intervene on behalf of our Saudi “allies.” With allies like this, we need no enemies, given lavish Saudi support for Islamism (and terrorist groups) around the world: indeed, I consider the Iranians’ in-your-face chants of “Death to America” more palatable than the Saudis’ two-faced duplicity. The relationship between the US and KSA is transactional, at best, and is unfortunately suborned by Saudi money which greases far too many palms in DC and Europe.
Stalemate is probably a good outcome from the US perspective. Getting in the middle means we will get it from both sides.
Our main interest is continued flow of oil through the Persian/Arab Gulf. A policy similar to that adopted by the Reagan Administration during the Iran-Iraq War, which largely took a hands-off approach to the conflict on the ground, and focused on assuring the free navigation of the Gulf, is a prudent one. If either side tries to escalate by attacking shipping or laying mines, like during the Tanker War, the US can intervene and smack them down as it did in Operation Praying Mantis.
We have no interest in a civilizational and sectarian war, and probably couldn’t intervene effectively even if we decided to. Neither country is capable of achieving a decisive victory over the other. The main stakes are who gets to rule (albeit indirectly) over a ruined Syria and a dysfunctional Iraq. So limit our involvement to keeping the oil flowing and deterring and preventing terrorist spillover. And definitely don’t take the side of Wahhabi freaks, or think that they are allies worthy of the name.