Facilities used by Iranian-backed militias in Syria were attacked by missile-launching aircraft. The Syrian Observatory for Human Rights said 22 fighters were killed. The air strikes were said to be in retaliation for attacks in Iraq by Iranian-backed Iraqi militias there.
Asked on Saturday what message he was sending to Iran, Biden said: "You can't act with impunity. Be careful."
With changing energy priorities and less reliance on Middle East oil these days, the US would prefer to focus more on China and Asia, and Russia and Europe. Even so, the desert sands keep drawing the US back in.
Biden has been sending plenty of messages.
He released an intelligence report that says Saudi Crown Prince Mohammed bin Salman approved the killing of journalist and dissident Jamal Khashoggi in 2018.
Yet new US visa restrictions introduced on Saturday for 76 Saudis do not directly target the prince. Some US commentators say the mastermind of the gruesome killing should be clearly punished.
New York Times columnist Nick Kristof wrote that Biden had "choked", adding that the "weak message to other thuggish dictators considering such a murder is: 'Please don't do it, but we'll still work with you if we have to'."
Biden has said he is dealing with the Saudi King rather than the prince. The President had previously ended US support for the Saudi-led conflict in Yemen and stopped some arms sales to its ally. That suggests an effort to isolate the prince. But the 35-year-old essentially runs the government.
It appears a lost opportunity for Biden to back words with consequences. More arm-twisting could be going on behind the scenes.
The President seems to want to return to the Obama Administration's approach of counter-balancing powers in the region. Under the Trump Administration, Saudi Arabia, Israel and Turkey gained in influence.
Israel normalised ties with the United Arab Emirates, Bahrain, Sudan and Morocco. Yemen slid further into disaster and Libya became the new Syria with its civil war drawing in thousands of outside fighters, including from Russia and Turkey.
Biden's air strikes were criticised by some members of the US Congress, which is meant to approve military action. Executive power for such actions has been regularly used in the "war on terror".
Other countries - Russia, Israel and Turkey - also regularly conduct strikes in Syria, which will mark 10 years of civil war this month.
Both the US and Iran are engaged in chest-beating ahead of a likely resuscitation of the nuclear pact with Tehran. Neither wants to look weak. Iran was complying with its obligations under a nuclear pact when the US under Donald Trump withdrew from it and imposed sanctions under a "maximum pressure" campaign.
An optimistic scenario could see improvement on the US-Iran nuclear issue opening the way for diplomatic progress on Syria, which would need to involve talks and cooperation between the US and Russia.
America's ongoing presence in the region makes it both a target and contributor to conflict. As ever, there are no easy answers in the Middle East.
