Behind recent U.S. and Iranian military mobilizations lies a perception by each side that the other may be about to attack. This doesn’t appear to be posturing by either side but is instead a confrontation that could led to actual conflict if there’s a miscalculation.

The United States sent an aircraft carrier task force, bombers and other assets into the region last weekend after officials concluded that Iran had altered its strategy of waiting out the Trump administration’s pressure campaign — and was instead making preparations for a possible military strike on U.S. forces in the region.

Marine Gen. Frank McKenzie, the new U.S. Central Command leader, said in a speech Wednesday that the buildup came “in direct response to a number of troubling and escalatory indicators and warnings.” His spokesman, Navy Capt. Bill Urban, had said the previous day there were “clear indications that Iranian and Iranian proxy forces are making preparations to possibly attack U.S. forces in the region.”

This sense of an imminent Iranian threat marks a break from what U.S. officials had predicted just two weeks ago. At that time, officials expected that Tehran would try to ride out President Trump’s campaign of sanctions over the next 20 months, in the hope they would be removed by his successor.

But last week, based on new information, the United States concluded that the Iranians had decided to reset their strategy now and were moving military equipment to prepare for action. It’s not clear whether this turnabout happened because U.S. sanctions were squeezing so hard that the Iranians couldn’t wait until January 2021, or because they concluded that Trump might be reelected.

The message of U.S. willingness to use force appears to have registered with Iran and its proxies. I received a text message Thursday morning from an American who travels widely in Syria and Iraq. He said he was contacting me on behalf of the head of the leading Iran-backed Shiite militia organization in Iraq. According to this intermediary, Iran-backed forces had “pre-positioned themselves to respond to U.S. escalation,” but “there was no plan for attack, only a response if the U.S. attacked.”

The militia leader’s tone seemed conciliatory: “There is still an opening to de-escalate things if the other side doesn’t want direct conflict,” he said, according to the intermediary who had been asked to speak on his behalf. The American intermediary, whom I’ve known for nearly a decade, conveyed the message on condition that the militia leader’s name and organization wouldn’t be directly identified.

U.S. officials have been particularly worried about a possible attack by Iranian proxies on the more than 5,000 U.S. troops in Iraq, who are training Iraqi military forces and otherwise bolstering security in the country. So the message from the Iranian militia leader speaks, at least indirectly, to a major American concern.

The Iranian-backed militias were said to have been especially worried after U.S. helicopters dropped flares near Camp Speicher in Iraq, near Tikrit, where some of the Shiite militias are based. The flares ignited fields of crops near the base, and the militias apparently feared that this might be a prelude to military action.

The larger question emerging from this week’s showdown is where the U.S. strategy of “maximum pressure” is heading. The Iranians clearly are feeling the squeeze and looking for a way to push back.

Part of Tehran’s pushback was this week’s announcement by President Hassan Rouhani that Iran would withdraw from parts of the 2015 nuclear agreement, in response to Trump’s announcement that he was abandoning the pact and the reimposition of U.S. sanctions. The Pentagon has feared that the Iranian reset might include kinetic military action, too, in the hope that it would push the United States back toward negotiations.

Iran’s confusion about the Trump administration was exemplified by a comment Thursday by Majid Takht Ravanchi, Iran’s ambassador to the United Nations, in an interview with NBC’s Andrea Mitchell. She asked whether Rouhani would be willing to sit down with Trump and negotiate a broader agreement, which Trump has sometimes said was his goal.

“All of a sudden, he said that ‘I don’t like this and let’s sit down and talk about another rounds of negotiations,' ” Ravanchi said. “What is the guarantee that he will not renege again on the future talks between Iran and the United States?”

What’s clearest, after this week of saber rattling, is that Iran was mistaken if it hoped that a show of force would lead the United States to retreat. McKenzie put it bluntly in his speech Wednesday: “Any attack on U.S. interests will be met with unrelenting force.” That doesn’t sound like a bluff.

Read more: