a) Iran wants a sphere of influence and not buffer zones.
It already has a sphere of influence, and to maintain that sphere of influence it needs a boogeyman, a local one that everyone can see and feel threatened by.
The Iraqis were silenced by the excuse of fighting terrorism.
Lebanon's Hezbollah and allies are using fighting terrorism as an excuse to stay in power.
Syria is the same.
b) Terrorists can't stop a regular army from passing through; they just stop a country from being pacified.
Insurgencies and militias can inflict attrition using asymmetric warfare and through disrupting supply lines.
c) Even Trump isn't stupid enough to attack Iran. The Iranians know that because they wouldn't have provoked the US by knocking out half of Saudi oil capacity or by capturing tankers in the Strait otherwise.
The US and Iran are already fighting a cold war against each other.
They don't attack each other directly.
d) Even if the US were to attack, it would be primarily from the sea and the US has enough bases in the region from which to attack. Even today it can move its troops in Syria with impunity.
First, it would need to attack through the plains in southern Iraq, it can't deploy sufficiently large army through the sea.
And second, if the US went to war, then its army will be attacked by the dozens of militias on the ground before reaching Iran.
e) Iran doesn't have the economic means to continuously sustain foreign wars.
It doesn't have the economic means to sustain foreign wars and social development and welfare together. It, however, can sacrifice one to sustain the other. Guess which one it's sacrificing?
f) Turkey, KSA, Qatar, the US/UK supported the rebellion in Syria.
True, but Iran is the one doing most of the fighting on the ground, and Iran is the one choosing not to end these groups, rather keep them alive.
There was no need for Iran to support ISIS in order to destabilize Syria.
It didn't have to support it, all it had to do is leave some space open for it to grow and not kill it off right away.
g) No country wants wars on its doorstep.
Yet when war is brought to your doorstep, you have to fight it.
h) Syria needed Iran because the Russians wouldn't have sent enough ground troops
And Syria wouldn't have needed Iran if these Islamists hadn't grown so strong.
The Syrian government actively released Islamists at the early stages of the rebellion in order to divide the rebels and make them fight each other, and when Iran got involved, it actively left space for them to grow and abstained from fighting on several occasions in order to let them expand and give it an excuse to increase its militias' size and its own military deployment.
j) You haven't provided a thread of evidence showing that Iran supported ISIS.
I didn't say that Iran supported ISIS.
I said Iran has an interest in keeping ISIS alive and well and that I wouldn't be surprised if it supported it.
We do however know that Iran does occasionally supply the Taliban, so we do know that it does use this strategy.
k) Iran doesn't need to "weaken its allies" because US/KSA are doing that.
It does, because strong allies at the moment would challenge it and effectively become rivals themselves.
Before the war, Iran had to negotiate with Syria for pipeline and trade deals for example, with gulf states like Qatar competing with Iran in these negotiations; Now, Syria doesn't have that choice and Iran can simply enforce its well.
l) Iran is supporting the Houtis in Yemen. That is to create a zone of influence and not a "buffer zone" to deter a US attack.
The Houthis are in Yemen, they act to surround Saudi Arabia and as a leverage card to shut naval supply lines passing through the red sea.
Any invading army attacking Iran, especially if it's a NATO one, would need to deal with the Houthis in order to secure its supply lines, making Yemen an effective buffer zone.
m) The danger to Iran are popular uprisings at home and in its sphere of influence and not a US invasion. The more it has to spend on foreign wars, the less there is for economic improvements to pacify popular uprisings.
The danger of an American invasion was very real for the past decades.
And now that an American invasion is less likely, Iran needs another, more immediate, foreign threat as an excuse to crack down on any descent and uprising.
ISIS serves that purpose just fine and thus we see that Iran routinely held back in fighting it.
And the main reason for the lack of economic development in Iran is corruption, not lack of resources.
I can imagine that the ME is a hot house for conspiracy theories.
This isn't a conspiracy theory, this is a simple analysis of Iran's interests and strategy in the region based on 1- historical precedents, and 2- popular strategies used by various governments in the region.