WARSAW — Arriving in Poland to host an international conference on Middle East peace and security, Secretary of State Mike Pompeo announced Tuesday that more than 30 foreign ministers would attend.

As a measure of its prospects, however, many countries have signaled that they will not be sending their top diplomats to the meeting — possibly nearly half.

As Pompeo and Vice President Pence prepare to welcome representatives of about 60 nations to Poland, co-host of the event, it is unclear what can be accomplished in a day of meetings over conflicts that have roiled the region for many years.

What Pompeo originally billed as a major conference to pressure Iran on its regional influence, missile testing and terrorism is now as likely to be defined by what it is not — and who is not coming. Several key countries appear to be engaging in a subtle diplomatic snub to protest the Trump administration’s policies toward Iran and Syria.

Under President Trump, United States-Iranian relations have taken a decisive turn for the worse. Here's a brief history of the tumultuous relationship. (Joyce Lee/The Washington Post)

Scratch Federica Mogherini, the foreign policy chief for the European Union, who said she had a prior commitment. France and Germany are sending second-tier-level diplomats. Russia won’t be there at all. And the British foreign secretary may leave early.

The muted response to an ambitious White House endeavor is partly due to concern that Pompeo and Pence would unleash a full-throttle rhetorical attack on Iran. Europeans have created a special barter-type trading system to work around U.S. sanctions reimposed on the country after Trump withdrew from the 2015 nuclear deal last year.

After some countries expressed concern about the anti-Iran focus of the conference, Pompeo telephoned several of his counterparts urging them to attend, according to foreign officials.

But since Pompeo’s announcement of the conference during a trip to the Middle East last month, the administration has backtracked to a degree on what it aims to accomplish. The acting U.S. ambassador to the United Nations, Jonathan Cohen, explicitly told the U.N. Security Council it is not “a venue to demonize or attack Iran.”

Instead, the agenda has expanded to include discussions on the war in Yemen, the Syrian civil war and the Israeli-Palestinian conflict. Despite the extra items, the two-day conference has meetings only on Thursday, with Wednesday’s sole event a welcome dinner.

Pompeo shrugged off the foreign minister no-shows and said he hopes much progress can be made. President Trump’s son-in-law and adviser, Jared Kushner, will offer hints of the status of a peace plan he has worked on for almost two years, with a preview of the rollout that has been delayed until after Israeli elections this spring.

“I think we’re going to deliver really good outcomes,” Pompeo said. “Some countries are having their foreign ministers come. Other countries are not. That’s their choice. But this is going to be a serious concrete discussion about a broad range of topics that range from counterterrorism to the malign influence that Iran has played in the Middle East towards its instability.”

Senior officials in the administration see reining in Iran as a necessary step in bringing about a broader Middle East peace plan that helps integrate Israel into the region. Israel considers Iran its major threat, and most Persian Gulf countries see the Shiite nation as their chief rival for influence.

The administration has been pressuring Arab allies, particularly in the Gulf, not only to support the still-undisclosed plan — which Trump has characterized as the “deal of the century — but also to persuade the Palestinians to accept it. One pressure point would be for the Gulf states to indicate they are prepared to establish a relationship with Israel.

Saudi Arabia, several other Gulf countries and Israel will be at the conference. The Palestinian Authority and Lebanon, where Iranian-backed Shiites hold political power, will not. Neither will Iran, which has denounced the conference as a “desperate anti-Iran circus.”

That suggests that one measure of a successful outcome would be a photograph of Israeli Prime Minister Benjamin Netanyahu together with Gulf Arabs.

Mark Dubowitz, head of the Foundation for Defense of Democracies and a critic of the nuclear deal with Iran, said it is advantageous to bring more countries on board to counter Iran. He said it should not be the sole province of the “P5 plus 1” countries — the United States, Britain, Russia, France and China, plus Germany — that negotiated the nuclear agreement with Iran.

“I think we got too used to the idea during the Obama administration and the Bush administration that the only way to solve the challenge of Iran is through six countries,” he said. “And that six countries need to be at the adult table, and everybody else is at the kids’ table.

“The P5 plus 1 was leading to diplomatic paralysis.”

A head of the no-show ministers could underscore the divisions between the United States and its European allies over Iran. Standing beside Pompeo at a news conference Tuesday evening, Polish Foreign Minister Jacek Czaputowicz pointedly said Poland considers the nuclear deal with Iran “a valuable element in the international arena.”

“The United States has a different opinion,” he acknowledged, but added: “That will not hinder us in looking for a common approach.”