The U.S. is countering Iran with diplomatic and economic actions rather than military confrontation. U.S. economic sanctions are forcing down the value of Iranian currency, which will immediately hurt the buying power of Iranian consumers. Iran is fighting back with a hollow threat to shut the Strait of Hormuz and cause an oil price spike. That would be suicidal for Iran because most of its oil is produced in Khuzestan province and must transit the Strait of Hormuz to get to Asian markets. The U.S. recognizes Iran's strategic weakness and likely relishes the pain its actions inflict upon Iran as retaliation for the capture of an RQ-170 drone. Iran presently presents rhetoric and not aggression because it has not yet exhausted its financial reserves.
The CNN news item linked above about the Strait threat briefly mentions the relative percentage of GDP the U.S. and Iran commit to their military budgets. That is an amateurish analysis. Relative force arrays and capabilities each side brings to bear in the Strait of Hormuz are much more relevant. Iran's navy cannot match a U.S. naval carrier battle group in a direct confrontation. The last direct U.S.-Iranian naval encounter, Operation Praying Mantis, was a limited engagement but ended in a lopsided victory for the U.S. Iran would thus need to rely upon standoff weapon systems in any effort to neutralize U.S. warships. Iran may possess some number of Sunburn anti-ship missiles or other similar systems. It would thus be imperative for U.S. national reconnaissance efforts to locate all of Iran's potential missile launch platforms and designate them as preplanned targets prior to hostilities. The U.S. has outstanding ISR capabilities; Iran's comparable ISR is questionable, with or without a purloined RQ-170.
The U.S. is a responsible world actor. Its Navy is professional enough to monitor Iran's declared military exercises from international waters without provoking a reaction. Iran is not nearly as mature. Any miscalculation that risks open hostilities is much more likely from Iran, given its economic deprivation and relative weakness, than from the U.S.