From what I understand, the ship is NOT an American ship and does not have any Americans on-board.  It's not clear where exactly the ship was encountered/engaged by the Iranian navy.

 

Technically speaking, a ship going through the Strait of Hormuz will in fact travel through Iranian territorial waters.  That's not really enough to justify the Iranians for boarding it or stopping it.

 

My guess is that Iran is trying to "flex their muscles" after their naval warships and a convoy or suspected weapons were basically "turned around" with the presence of superior U.S. Navy forces in the area.  I'm willing to bet that once a U.S. Navy destroyer gets into the area, the ship will be released and escorted out of Iranian waters.

 

This was simply a ploy for Iran to spread more propaganda in their country by saying that they fired on and seized a U.S. ship.

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