James Stavridis’ argument that Iran is an imperial power relies heavily on very superficial analysis:
We don’t tend to think of today’s Iran as an imperial power, but the Iranians certainly do — indeed, it is woven into their national DNA and cultural outlook.
Whenever someone starts a foreign policy article about another country by talking about a certain trait being “woven into their national DNA,” it’s a safe bet that the author is making a very biased and tendentious argument. Debating Iran policy isn’t the only occasion when analysts indulge in such crude stereotyping and lazy essentialism, but it is more common in this debate than in many others. Stavridis points out that there have been powerful Persian empires that have dominated large parts of the Near East and then asserts that something similar is happening again today. The evidence for contemporary Iran’s “imperial ambitions” is very thin, especially when it comes to Tehran’s ability to realize such ambitions.
Stavridis leans heavily on the assumptions behind the “march of conquest” narrative popularized by Netanyahu:
Iran is deeply and successfully dominating politics in the capitals of four major states in the region from Beirut to Baghdad, Sanaa to Damascus.
It would be much more accurate to say that Iran has significant influence in three of these capitals, but it grossly exaggerates the degree of Iranian power and control to say that they are dominating the politics of these states. The claim about Yemen is extremely misleading, since it accepts at face value the notion that the Houthis are little more than an extension of Iranian power. Even when the government relies on Iranian support against its internal enemies, as it does in Syria and Iraq, that means that Iran is being forced to waste resources to prop up its allies and clients. To the extent that Iran has influence in Damascus and Baghdad, it is being bought at a high price. If we look carefully at Iran’s role in the region, we see that it has been limited to being little more than the patron of Shi’ites and Alawites, and that puts it at odds with most of the region’s governments. 
That may cause the Saudis and other Gulf states alarm for their own sectarian reasons, but it should tell us that Iran’s regional reach is very limited.