Trump was deliberately ambiguous in his threat against Iran

NY Times:

Trump’s Threat Against Iran Was Loud, but Far From Clear

• President Trump’s tweet in all capital letters, which echoed his approach with North Korea, succeeded in changing the subject after a week of bad headlines about his meeting with President Vladimir V. Putin of Russia.
• But it only deepened questions about Mr. Trump’s Iran policy and left the White House scrambling to lend a veneer of coordination to the outburst.

Sometimes ambiguous threats are more effective than specific threats.  For example, when Obama made his "red line" threat against Syrian use of WMD he put himself in a box that required him to actually do something he did not want to do or have to do a climb down.  If he had instead indicated that there would be unpleasant consequences for the regime is they did use WMD he would have had more flexibility in how he responded when they did.  That is essentially what Trump did in responding to Iranian bellicosity.