Republican Senator Lindsey Graham has criticized President Donald Trump’s response to the shooting down of an American drone by Iran in June, when Trump reportedly called back a retaliatory military strike in the last minutes.

Graham tweeted on September 17 that Trump’s “measured response” to the shooting down of the drone “was clearly seen by the Iranian regime as a sign of weakness”.

In the wake of the devastating attack on Saudi oil installations on Saturday, some have questioned whether Iran has become emboldened as a result of a lack of American military response to the June attack.

Graham is an ally of Trump and his tweet was a careful criticism in a mild tone. However, hours later Trump responded with a tweet of his own saying, “No Lindsey, it was a sign of strength that some people just don’t understand”.

Reporters and observers have occasionally commented that president Trump prefers not to enter a military conflict with Iran and his weapon of choice is economic pressure. Iran might have realized this and decided it can adopt an aggressive policy trying to show its military power to intimidate the U.S. and its allies in the region.

Iran's economy is under tremendous pressure as a result of U.S. sanctions and it has threatened that it can make oil exports and shipping in the region difficult for others.