Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. His new book is "Trump and His Generals: The Cost of Chaos." The opinions expressed in this commentary are his own. View more opinion articles at CNN.

(CNN) Early Sunday, the United States conducted airstrikes against five targets in Iraq and Syria associated with an Iran-backed militia. The militia, Kataib Hezbollah, has been blamed by the Pentagon for a number of recent attacks on American forces in Iraq, such as an attack Friday on a base near Kirkuk that killed a US civilian contractor and wounded four American servicemen.

The Pentagon was careful to note in the statement it released Sunday about the US strikes on the Kataib Hezbollah targets that these actions were "defensive" in nature because they were in response to attacks on American servicemen and contractors.

Peter Bergen

The military action, in conjunction with prior hesitation by the Trump administration to attack Iranian targets, suggests an emerging Trump doctrine about when he will authorize the use of military force.

Earlier this year, President Trump was hesitant to use force against Iran after Iranian forces shot down an American surveillance drone. At first, military action seemed probable, but on June 20 he pulled back airstrikes on Iranian targets that could have killed as many as 150 people, claiming it wasn't a "proportionate" response to the unmanned drone that was brought down.

Three months later, on September 14, a barrage of missiles and drones targeted two of the world's most important oil facilities in Saudi Arabia, knocking out about half of the country's oil capacity and immediately spiking oil prices around 15% . A day after the attacks in Saudi Arabia, President Trump tweeted that the United States was "locked and loaded depending on verification" of who was behind the attacks. Yet when the Trump administration credibly blamed Iran for the attacks, Trump, it turned out, didn't want to get embroiled in another war in the Middle East and chose to impose additional sanctions on Iran instead of a military response to the attack on a close ally.

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