Editor's Note: (David A. Andelman, a contributor to CNN, twice winner of the Deadline Club Award, and executive director of The Red Lines Project, is the author of "A Red Line in the Sand: Diplomacy, Strategy, and the History of Wars That Might Still Happen" and host of its Evergreen podcast. He formerly was a correspondent for The New York Times and CBS News in Europe and Asia. Follow him on Twitter: @DavidAndelman. The views expressed in this commentary belong solely to the author. View more opinion at CNN.)
(CNN) In a single stroke on Thursday, President Joe Biden did far more than destroy a few support facilities for Iranian-backed Syrian militias. Unlike his two predecessors, Biden has drawn a new red line in the Middle East and shown he is prepared to defend it.
Pentagon officials said two F15s dropped Joint Direct Attack Munitions (precision weapons with GPS navigation capabilities) on buildings at an unofficial border crossing believed to be used by militias to transfer munitions into Syria from Iraq.
The strike sought to cripple the ability of these groups, particularly Kata'ib Hezbollah and Kata'ib Sayyid al Shuhada, to attack American forces, Pentagon spokesman John Kirby said.
It also sent a stark message to Iran: That it can no longer use militias in Syria and Iraq as proxies to attack Western, especially American, interests anywhere in the region. And that Biden is ready to punish any violations of this new red line surely and quickly.
"It's a direct signal to the Iranians that there will be responses to the actions of their front groups in Iraq, which are, in part, run by Kata'ib Hezbollah," Phillip Smyth, a researcher at the Washington Institute for Near East Policy and an expert in Shia militant groups, told me in an e-mail. "It's a middle path of reaching out and touching someone. It's also a good way to test how the Iranians are dealing with the U.S. in Iraq."
Biden's move is a sharp departure from his predecessors Donald Trump and Barack Obama. Trump blustered about America's military might and pulled out of the deal the Obama administration struck with Iran restraining Iran's push to develop nuclear weapons, at least in part because it put no restraints on Iranian support for Shiite militias.
But he also drew down American forces in the region to the lowest levels since before the Iraq war. Obama declared a red line in Syria against the use of chemical weapons, then failed to act when the regime of Bashar al-Assad crossed it.
By striking at militia operations in Syria, rather than in Iraq where the munitions convoys originate, Biden was also showing sensitivity to the fragile regime in Baghdad of Prime Minister Mustafa al-Kadhimi, with whom he talked on Tuesday. The White House said in a statement afterward that they both agreed "that those responsible for such attacks must be held fully to account."
Biden's first military action was measured and direct. The administration says it remains committed to discussing a return to the nuclear accord and perhaps expanding those talks to include Tehran's other activities in the region.
If the administration's posture in the region remains one of reasoned but consistent toughness, Iran may restrain itself from any tit-for-tat retaliation and response. It may now understand the price it could pay for failing to respond to US overtures at diplomacy -- and continuing to back those militias.