Brussels (CNN) President Donald Trump on Thursday will attend his first NATO summit alongside the 27 other members of the military alliance and join in the opening ceremony of the group's new headquarters in Brussels.

The day's events are highly anticipated after his election last November shook members of the NATO military alliance, since as a candidate he repeatedly decried the military alliance.

Calling it "obsolete," Trump said its members weren't paying their fair share and cast doubt on US commitment to defending NATO allies should they come under attack -- one of the alliance's central tenets.

Here's what to watch as the day unfolds:

Reassuring allies

NATO leaders and member countries are eagerly awaiting Trump's remarks, seeking reassurance about the US' commitment to the military alliance.

Trump's campaign comments questioning NATO's relevance deeply unnerved member states, particularly those at Russia's doorstep that count on US support to ward off Moscow's increasingly aggressive posturing.

So far, Trump appears on track to delivering those assurances.

He has already backed off his campaign trail claim that NATO is an "obsolete" organization, declaring during a joint news conference with NATO Secretary General Jens Stoltenberg at the White House last month that the alliance "has been the bulwark of international peace and security."

"I said it was obsolete. It's no longer obsolete," Trump said.

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But as Russia continues to maintain its aggressive posture in Eastern Europe, many NATO members are also waiting for Trump to reaffirm US commitment to the lynchpin of the alliance: Article 5.

Under Article 5, NATO members promise to jump to the aid of any member country that is attacked, declaring that an attack against one of the 28 countries is an attack on all. The alliance has only once invoked Article 5: the day after the terrorist attacks of September 11, 2001, which triggered NATO's participation in the war in Afghanistan.

Trump left NATO members shell-shocked last summer when he suggested the US might only defend members who met their financial commitments, saying only that he would stand by NATO allies "if they fulfill their obligations to us."

Briefing reporters Wednesday on Air Force One, Secretary of State Rex Tillerson said "of course we support Article 5," but declined to say whether Trump would reiterate the US' adherence to the mutual defense pledge in his NATO remarks.

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Financial commitments

While Trump has offered reassurance, he has also made clear that compelling members to boost their financial commitment to the alliance remains a top priority.

In joining the alliance, member states pledge to spend 2% of their GDP on defense. But only a handful of the 28 countries in the alliance actually meet that target.

And Tillerson said Wednesday to expect Trump's calls for countries to boost their defense commitments to be "the core of his message to NATO" on Thursday.

"I think you can expect the President to be very tough on them, saying, 'Look the US is spending 4%. We're doing a lot. The American people are doing a lot for your security for our joint security. You need to make sure you're doing your share for your own security as well,' " Tillerson said.

Trump is not the first US president to call for NATO allies to meet their financial commitments to the alliance, but he has put a bullhorn to the issue and both he and Stoltenberg have continued to pressure member states to increase their defense spending.

While it's not clear that he'll raise the issue in public, Trump will meet with several allies behind closed doors, including during a working dinner Thursday evening with all countries in the alliance.

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Russia

Trump is calling for the alliance, which was initially assembled during the Cold War to ward off the Soviet Union, to refocus itself on fighting terrorism.

But as Russia continues to maintain its aggressive posture in Eastern Europe and the Baltic on the heels of its annexation of Crimea and amid its continued troop presence in Eastern Ukraine, NATO members remain on edge and on guard.

Following its interference in the 2016 election, Russia has continued to attempt to influence elections in other NATO member countries, including in France , through cyberhacking and propaganda tools.

Trump during the campaign vowed to improve US-Russia relations, but Trump declared last month that US-Russia relations "may be at an all-time low."

But Trump has raised fresh concerns in recent weeks as he's sought to engage Russia, inviting the Russian Foreign Minister Sergey Lavrov to the Oval Office where sources said Trump shared highly classified information that a key US ally had provided on a confidential basis.

As Trump walks into the newly minted NATO headquarters on Thursday, he remains dogged by a cloud of suspicion over the federal investigation into contacts between his campaign associates and Russian officials during the election.

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Afghanistan

But US officials have signaled in recent weeks that the Trump administration would like to see an increased commitment from NATO, which has a total of 13,000 troops currently deployed to Afghanistan, roughly half of which are American.

The Pentagon recommended the troop increase weeks ago, but Trump has been waiting to gauge the commitment of fellow NATO allies before making a decision.

Tillerson said Wednesday that "the Afghan policy review is still a work in progress."

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ISIS

While several NATO allies have joined the US in the coalition fighting ISIS in Iraq and Syria, the alliance has yet to join the fight.

Trump has stressed that he would like to see NATO redouble its efforts in fighting terrorism, which Trump sees as one the greatest threats to the West, and said last month he hopes "NATO will take on an increased role in supporting our Iraqi partners in their battle against ISIS."

Tillerson said joining the ISIS coalition "would be a really important step for them to take" and noted that the alliance has already "become more and more engaged in the actual fight to defeat ISIS."

"There are a couple of countries that are still thinking it over," Tillerson said. "I think they're going to support NATO joining and becoming" a formal member of the anti-ISIS coalition."