trump administration is preparing to reinstate sanctions against iran and president trump and iranian leaders have spent the past few weeks exchanging threats on social media. cnn's international diplomatic editor nic robertson joining me right now. nic, no accident that iran has chosen the strait of hormuz to flex its military muscle. this is a major shipping area. what is the message being sent? >> really seems to be the message being sent that if iran so chooses, it can disrupt the world's oil fields. 20% of the world's oil passes through there. it's a major choke point effectively. and impacting one oil tanker, never mind shutting down the whole straits, would put a spike, a nasty spike into oil markets. perhaps trigger higher fuel costs for us. the reality is we don't know why iran has put these military