Iran’s government now is threatening to close the Strait of Hormuz if the US joins its war on Israel on Israel’s side, among other things by bombing Fordow and Natanz with MOPs.
Iran certainly could, but for how long? My prediction is for a few hours to a few days, at the end of which Iran would have left no navy worthy of the name and no Arabian Gulf or Arabian Sea ports of any use to the remnants of its navy or to its commercial shipping—including is ghost tanker fleet with which it ships embargoed oil.
The reasons center on Iran’s own incapacity. It has no air assets with which to close or to keep closed the strait; it has only a supply of cruise missiles which it would have to divert from its attacks on Israel to close and keep closed the Strait, and its small navy with which to sail the strait.
That navy and the ports from which it would sally are nothing more than targets for the US Navy, which has been expanding in the region, and it would be a campaign of a matter of hours or a few days to sink the Iranian navy and destroy those ports. A few destroyers would serve to protect commercial shipping in the strait, and in the Gulf, come to that, against those cruise missiles, just as those destroyers and cruisers have done in the Gulf of Aden.
There would be sequelae to an Iranian attempt to close the strait. At the end of the campaign to reopen the strait, Iran would have limited capability to get its oil onto tankers (it would be useful for the reopening campaign also to sink such ghost tankers as happen to be in the Gulf or the nearby Arabian Sea, which would further restrict Iran’s ability to ship its embargoed oil). That would hurt the People’s Republic of China’s economy, which has been importing lots of price-discounted Iranian oil (discounts the PRC can demand since it takes 90% of Iran’s oil exports and so can demand the discounts).
Another consequence would be further reductions in Iranian (re)supplies to the Houthis.
There would be a spike in oil prices from a closure of the Strait of Hormuz, but that would be temporary, and the closure would lead, nearly inevitably, to those follow-on sequelae, which would be to the net good of the larger world.