BRUSSELS (AP) — President Joe Biden reaffirmed the U.S. commitment to NATO on Monday as leading members declared it a pivotal moment for an alliance beleaguered during the presidency of Donald Trump, who questioned the relevance of the multilateral organization.

Shortly after arriving at the alliance's headquarters for the first NATO summit of his presidency, Biden sat down with NATO Secretary General Jens Stoltenberg and underscored the U.S. commitment to Article 5 of the alliance charter, which spells out that an attack on one member is an attack on all and is to be met with a collective response.

“Article 5 we take as a sacred obligation,” Biden said. “I want NATO to know America is there.”

It was a sharp shift in tone from the past four years, when Trump called the alliance “obsolete" and complained that it allowed for “global freeloading” countries to spend less on military defense at the expense of the U.S.

Looking forward, Stoltenberg noted myriad challenges still facing the alliance.

“We are meeting at the pivotal time for our alliance, the time of growing geopolitical competition, regional instability, terrorism, cyber attacks and climate change," Stoltenberg said at the start of a joint session of the NATO leaders. “No nation and no continent can deal with these challenges alone. But Europe and North America are not alone."

Biden, who came to Brussels following three days of consultations with Group of Seven leaders in England, was greeted by fellow leaders with warmth and even a bit of relief.

Belgium Prime Minister Alexander de Croo said Biden’s presence “emphasizes the renewal of the transatlantic partnership." De Croo said NATO allies were looking to get beyond four stormy years under the Trump administration and infighting among member countries.

“I think now we are ready to turn the page," de Croo said.

Italian Prime Minister Mario Draghi made a not-so-subtle dig at Trump, while welcoming Biden.

“This summit is a continuation of yesterday’s G-7 and is part of the process of reaffirming, of rebuilding the fundamental alliances of the United States that had been weakened by the previous administration,” he said. “Think that President Biden’s first visit is to Europe and try to remember where President Trump’s first visit was?"

Trump's first overseas visit as president was to Saudi Arabia.

Trump routinely berated other NATO countries for not spending enough on defense and even threatened to pull the U.S. out of the world’s biggest security organization and even questioned the mutual defense provision of the NATO charter, a central tenet of the alliance.

When alliance members last met for a summit in England in December 2019, Trump grabbed headlines by calling Canadian Prime Minister Justin Trudeau “two-faced” and French President Emmanuel Macron “nasty.”

Trump lashed out after Trudeau was caught on a hot mic gossiping with other leaders about Trump turning photo opportunities into long news conferences. Ahead of the summit, Macron had declared NATO “brain dead” because of a void in U.S. leadership under Trump.

The White House said the communique to be signed by alliance members at the end of the NATO summit is expected to include language about updating Article 5 to include major cyber attacks — a matter of growing concern amid a series of hacks targeting the U.S. government and businesses around the globe by Russia-based hackers.

The update will spell out that if an alliance member needs technical or intelligence support in response to a cyber attack, it would be able to invoke the mutual defense provision to receive assistance, according to White House national security adviser Jake Sullivan.

The president started his day meeting with leaders of the Baltic states on NATO's eastern flank as well as separate meetings with leaders of Poland and Romania to discuss the threat posed by Russia and the recent air piracy in Belarus, according to the White House.

Biden's itinerary in Europe has been shaped so that he would first gather with G-7 leaders and then with NATO allies in Brussels before his much-anticipated meeting with Russian President Vladimir Putin in Geneva on Wednesday. And with both summits, Biden aimed to consult European allies on efforts to counter provocative actions by China and Russia.

The G-7 meeting ended with a communique that called out, at the urging of Biden, forced labor practices and other human rights violations impacting Uyghur Muslims and other ethnic minorities in the western Xinjiang province. The president declined to discuss private summit negotiations over the provision, but said he was “satisfied” with the communique, although differences remain among the allies about how forcefully to call out Beijing.

The Chinese embassy to the United Kingdom on Monday issued a statement saying the communique “deliberately slandered China and arbitrarily interfered in China’s internal affairs," and exposed the “sinister intentions of a few countries, such as the United States."

Biden has said he's focused on building a more cohesive bond between America and allies who had become wary of U.S. leadership after enduring four years of Trump's name-calling and frequent invectives about NATO.

He has already acknowledged during his Europe tour that the alliance needs to ensure better burden-sharing and needs more American leadership. He’s also highlighted NATO members' contributions in the war in Afghanistan, noting that “NATO stepped up” after America was attacked on Sept. 11, 2001.

The U.S. and the alliance are winding down their involvement in the nearly 20-year war that killed tens of thousands of Afghans and more than 3,500 U.S. and allied troops. The war also raised profound questions about whether NATO’s effort was worth it.

For now, NATO plans to leave civilian advisers to help build up government institutions. It’s unclear who will protect them. The alliance is also weighing whether to train Afghan special forces outside the country.

Biden will meet later on Monday with Turkey's president, Erdogan, on the summit sidelines.

Biden has known Erdogan for years but their relationship has frequently been contentious. Biden, during his campaign, drew ire from Turkish officials after he described Erdogan as an “autocrat." In April, Biden infuriated Ankara by declaring that the Ottoman-era mass killing and deportations of Armenians was “genocide" — a term that U.S. presidents have avoided using.