WASHINGTON — Rallying the world against Venezuela and Iran, President Donald Trump is getting more traction targeting the adversary he never seemed to notice than the one he’s been fretting about for years.

A high-profile summit in Warsaw opening Wednesday and organized by Trump’s administration is highlighting the deep schism between the United States and other nations on Iran. But many of those same nations are embracing Trump’s call to action on Venezuela and his denouncement of embattled leader Nicolás Maduro.

The administration had hoped the Warsaw conference would be a showcase of global unity against Tehran, coinciding with the 40th anniversary of the Iranian Revolution. When it was announced, Secretary of State Mike Pompeo emphasized it would address “Middle East stability and Iran” — identifying no other country by name.

But three diplomats from NATO countries tell NBC News that America’s allies pushed back, with many nations telling the U.S. they would not participate if the summit was a narrow effort to gang up on Iran.

Amid the pushback, the Trump administration dropped all mentions of Iran from its description of the summit and announced other topics that would also be addressed, such as the Mideast peace plan being drafted by Trump adviser and son-in-law, Jared Kushner.

“Their idea from the very beginning was to have a conference on Iran, no doubt. But then they realized pretty soon that would be not possible because most countries wouldn’t like that,” one of the foreign diplomats said. “So they changed it.”

Although Pompeo says more than 60 nations are attending the conference, many key countries are staying away, including China and Russia — both members of the Iran nuclear deal. At the conference, which will be attended by Vice President Mike Pence and Israeli Prime Minister Benjamin Netanyahu, many nations are sending lower-level representatives such as career diplomats.

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The resistance follows widespread frustration by European nations about Trump’s withdrawal from the nuclear deal, which Iran and the deal’s other members have vowed to keep alive. Those nations have also refused to comply with the Trump administration’s requests that they slap new sanctions on Iran, vexing Trump’s efforts to ramp up a global “maximum pressure campaign.”

Trump’s challenges in projecting global unity against Iran also illustrate how his go-it-alone approach to foreign policy and frequent spurning of allies is impeding his ability to secure support overseas when it’s needed to advance his top national security priorities.

NATO members have been dismayed by his comments suggesting the U.S. might not remain in the alliance. Former Defense Secretary Jim Mattis was so distraught over the way the president disregarded U.S. allies with his Syria withdrawal decision that it contributed to his decision to quit.

“Europe’s approach to Iran is about the best example of Europe trying to assert its independence against U.S. policy,” said Heather Conley, a former U.S. diplomat and Europe expert at the Center for Strategic and International Studies.

In contrast, Trump’s administration has faced far less friction in persuading foreign nations and international institutions to follow America’s lead in Venezuela. Last month the U.S. became the first country to recognize opposition leader Juan Guaidó, head of the National Assembly, as the country’s legitimate leader in a sharp rebuke to Maduro, the Venezuelan president.

Two dozen nations have joined the United States in recognizing Guaidó in the three weeks since. The group includes major powers like Germany and the U.K. along with key regional nations including Brazil, Colombia and Argentina. Major international institutions such as the Organization of American States, the Inter-American Development Bank and the European Parliament have also joined the cause.

For Trump, the growing coalition backing Guaidó represents one of his most successful diplomatic initiatives to date, and has also been met by broad bipartisan support in the United States. Even Democrats who have criticized the president for suggesting the U.S. could take military action in Venezuela have generally supported his move to recognize the opposition leader and to push back on Maduro’s human rights abuses.

Yet if there’s an irony for Trump, it’s that his success has come on an issue that was never high on his agenda in the first place.

As a presidential candidate, Trump spoke incessantly about Iran and what he described as the horrors of the 2015 nuclear deal, struck by President Barack Obama and world powers, under which Iran agreed to limits on its nuclear program in exchange for billions of dollars in sanctions relief.

“They are bad actors,” Trump said of Iran during the second Republican presidential debate in September 2015. “Bad things are going to happen.”

Maduro’s bad behavior, on the other hand, had never been a focus of Trump’s until recently, and his strong stance of late has surprised many foreign policy experts who have wondered what’s driving the newfound interest. On the campaign trail, Trump’s mentions of Venezuela were limited to warning about Democrat Hillary Clinton’s “Venezuela-style politics of poverty” and arguing that the U.S. economy would soon look like Venezuela’s if she were elected.

In the case of Iran, European nations are so determined to undermine the Trump administration’s policy that they’ve been working to create an alternative financial transaction system, known as the Special Purpose Vehicle, to sidestep the U.S. and facilitate some business and humanitarian trade with Iran.

“They were disgruntled because they were just re-establishing business connections with Iran” that had become permissible under the nuclear deal, said Evelyn Farkas, a former Defense Department official and an NBC News national security analyst. “It’s a relationship that is useful to the Europeans from a commercial perspective.”

But the Trump administration has argued that even though many nations are refusing to go along with new sanctions, the administration’s pressure campaign is profoundly hampering Iran’s economy and hurting the government there. That’s because the threat of running afoul of the United States and facing penalties has led major businesses to independently decide to cut all commercial ties with Iran.

Mark Dubowitz, who runs the Foundation for Defense of Democracies, a think tank that advocates hawkish U.S. policies on Iran, said that Trump’s tough Iran sanctions would have been only marginally more effective had European governments joined him. But he said the bigger question was whether Trump’s administration could successfully build a coalition to pressure the country diplomatically.

“Obviously it hasn’t been as successful as it has on Venezuela and hasn’t been as successful as the Obama administration was,” Dubowitz said. “Having said that, I think Warsaw is going to be a pretty good test at how effective the administration has been at bringing countries to the table to discuss these issues.”