In the eyes of Mike Pompeo, the day was shaping up to be one of his most commanding displays of diplomacy since becoming secretary of state. Months of planning had finally yielded a meeting among reluctant European officials, Arab leaders and the Israeli prime minister to strategize over confronting Iran.

Addressing the officials as they met this month in Warsaw, Pence denounced the United States’ closest allies Britain, France and Germany for coddling “Iran’s murderous revolutionary regime.” He demanded they stop undermining U.S.-led sanctions and follow President Donald Trump in renouncing a nuclear deal the Europeans were trying to save.

U.S. Secretary of State Mike Pompeo accused Barack Obama on Thursday of sowing chaos in the Middle East by failing to adequately confront Islamist militants in a blistering critique of the policies of President Donald Trump’s predecessor.

Privately, Pompeo briefly erupted. Aides said he complained Pence had undermined diplomacy which one European official said included near-agreement about imposing new sanctions on Iran’s ballistic missile tests and prompted fresh headlines about trans-Atlantic tensions.

Almost a year into his job, Pompeo, 55, has managed to pull off what few other Trump-era senior Cabinet members have accomplished staying in the president’s good graces and wielding power without countering White House pronouncements or policies.

Now he faces his greatest test. This week, Pompeo will accompany Trump to a summit meeting in Hanoi, Vietnam, to try to keep an effort to rid North Korea of its nuclear weapons from going off the rails.

Pompeo must keep Trump, 72, from being duped by the North’s wily leader, Kim Jong Un, who is half the president’s age. He also needs to prevent the president from undermining the U.S. negotiating position on denuclearization. After meeting with Kim in June in Singapore, Trump declared that the matter was “largely solved” and that “there is no longer a nuclear threat” from North Korea contradicting U.S. intelligence agencies, whose judgments the president largely ignores. Pompeo told CNN on Sunday that the North remains a nuclear threat.

In private discussions with Korea experts, Pompeo has conceded that he would be lucky if the North agreed to dismantle 60 percent of what the United States has demanded. But he said even that would be more than any other administration has achieved.

Overall, Pompeo has been more of an evangelist for the president’s “America First” approach than any other Cabinet member. Whether he should serve as a megaphone for Trump and the White House, or be more strident in confronting the president over the uncomfortable realities of foreign policy, is now the central question of Pompeo’s leadership of the United States’ diplomatic corps.

After Trump abruptly declared in December that the United States would soon withdraw its 2,000 troops from Syria, the defense secretary, Jim Mattis, resigned. But Pompeo told aides the withdrawal was the president’s prerogative and defended it as a “change in tactics, not in mission.”

Still, he is not always in step with Trump. He has pushed ahead with sanctions against Russia, and in Poland, he watched American and Polish troops in a live-fire exercise near the Russian border an unsubtle message to Moscow not to test the North Atlantic Treaty Organization.

But generally, Pompeo, a strong partisan and ideologue, aligns with Trump. In a scathing nationalistic speech in Brussels in December, Pompeo criticized institutions the United States helped create to maintain its global power the United Nations, European Union, World Bank, International Monetary Fund and World Trade Organization.