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President Donald Trump's order for airstrikes on a Tehran-backed Iraqi militia group, after resisting retaliating against Iran for months, sent a clear message Sunday that killing Americans was his red line.

But experts warned that, far from being deterred, Iran might find that line signals there is space for them to continue the kind of provocative activities that fired up tensions across the Gulf region throughout 2019.

And with Trump facing a re-election fight in 2020, some said Tehran could even step up its actions to challenge the president's promise to pull US troops out of the Middle East.

US officials said Monday that Trump had exercised "strategic patience" during the past year in the face of Iran's stepped-up military activities in the region challenging the US and its allies.

But they said that the death Friday of a US civilian contractor in Kirkuk in a rocket attack by the Hezbollah Brigades, an Iran-supported militia, forced Trump's hand.

At least 25 members of the group were killed in retaliatory US strikes Sunday on five of their bases in Iraq and Syria.

"The president has shown a lot of restraint," Brian Hook, the State Department's Special Representative for Iran, told reporters Monday.

"We very much hoped that Iran would not miscalculate and confuse our restraint for weakness. But after so many attacks, it was important for the president to direct our armed forces to respond in a way that the Iranian regime will understand."

Iran 'pushed the envelope'

Trump mulled and then deferred retaliation against Iran several times this year over its attacks on foreign oil tankers, the downing of a US drone and the brash September drone-and-missile assault on oil plants in Saudi Arabia, which took out nearly half of Riyadh's oil output.

Each time, the US leader fell back on more economic sanctions, despite them having had little visible impact on Tehran's expansive regional military operations.

Since October, the Hezbollah Brigades, which the Pentagon said are supported by the Iranian Revolutionary Guard Corps, carried out some 11 rocket attacks on installations in Iraq where US and coalition forces are present.

But until this weekend the response has been minimal.

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What changed was the death of an American, which "pushed the envelope," State Department Assistant Secretary David Schenker said Monday.

"We thought it important to hit a significant target set to send a very clear message to them about how serious we take American lives," he said.

"This was a defensive action designed to protect American forces and American citizens in Iraq, and it was aimed also at deterring Iran," Secretary of State Mike Pompeo told Fox news.

"President Trump has been pretty darn patient, and he's made clear, at the same time, that when Americans' lives were at risk we would respond."

- 'Invidious choice' -
With Sunday's airstrikes, Schenker stressed that Washington aims to deter Iran, but does not seek to escalate the conflict between the two countries.

But experts warned that Tehran could receive a different message: that, as long as Americans aren't directly injured, it could boost its covert activities.

Richard Haass, president of the Council on Foreign Relations, said that Iran's growing political weight in Iraq and Trump's aim to reduce the Pentagon's footprint in the Middle East could well result in Baghdad pressuring the United States to pull out.

Martin Indyk, a former US ambassador to Israel, said Iran could sense an opportunity to further provoke Washington, knowing where Trump's red line is.

"The consequences of strikes against Iranian proxies going into a US election year is that the Iranians now believe they own the higher rungs of the escalation ladder," he said on Twitter.

"If US troops in Iraq come under attack Trump will have an invidious choice: another [Middle East] war or backing down."