The Strait of Hormuz is a ridiculously narrow channel through which about one-third of the world's seaborne oil is transported. Most of that oil goes to Europe or Asia. Our allies would exert heavy pressure on Washington to protect the tankers -- and to do so quickly and completely. To prevent a sudden economic shock of oil at $250/barrel, the USA would have its own reason to intervene.
Because Iran has a substantial military, NATO and Japan by themselves could not possibly keep Hormuz open. It would not be an easy task for the USA military, either. Slow-moving, large merchant vessels like tankers are easy targets for missiles. Numerous missile sites are scattered throughout Iran; as we remember from 1991, it takes time to identify and destroy them all. Unlike Iraq, Iran has a navy. It's not large, but it's not toothless and would have to be dealt with.
The irony, of course, is that the Iranian military-industrial complex was intentionally created by the USA during the 1960s and 1970s so that our surrogate the Shah could dominate the region. Remember the movie Apocalypse Now? In the dinner scene at the French plantation, it was mentioned that the USA created the Viet Minh, predecessor to the Viet Cong, to fight the Japanese in World War II. Déjà vu.
As perspective, Iran is nearly four times the physical size of Iraq and has more than twice the population. The USAF and USN would take casualties in keeping the Strait of Hormuz open, and such an operation would become an indefinite commitment. Will cool heads prevail? I hope so, but I'm far from certain of it.
