- Trump has backtracked on his claims that NATO is "obsolete"
- NATO allies want Trump to make a full commitment to collective defense principle of NATO treaty
Even before the atrocity, the US President wanted to push NATO allies to commit to a greater role in counter-terrorism and the fight against ISIS in Syria and Iraq. Monday's concert bombing will bring renewed urgency to that discussion.
European NATO leaders are approaching this summit with trepidation. During his campaign for the White House, Trump sent ripples of alarm through the continent when he described the 68-year alliance as "obsolete" because it did not do enough to tackle one of the greatest challenges facing the world: Islamist terrorism.
Though he has backtracked since becoming President by declaring it not obsolete, NATO allies in Europe remain concerned and want him to make a full commitment to Article 5 of the North Atlantic Treaty, which states that an attack on one member is deemed to be an attack on all.
Tied to this concern are the links between the Trump administration and Russia, the country seen as the greatest threat to European stability by NATO allies on this side of the Atlantic.
"It is not unusual for the first summit for a new US President to be a 'get to know you' summit -- but there is a bit more at stake," said Sophia Besch, a NATO expert at the Centre for European Reform.
European allies had to take Trump's "obsolete" comments "very seriously," Besch said, because Europe depends on America's defense capabilities and contribution to NATO -- US currently provides 22% of the organization's budget, while Germany gives 15% and the UK provides 10%.
Other NATO members have a target of 2% of GDP on defense spending, but only a minority have met this target. She added: "Trump has never formally committed to Article 5 because he doesn't accept that Russia may be a threat to Europe."
If other NATO states can show they are doing more on counter-terrorism and burden-sharing, Trump is expected to commit to the alliance. But the question of relevance still lingers.
NATO as an organization cannot prevent the kind of terror attack that took place in Manchester, Besch said, but can focus on tackling ISIS in the Middle East.
"We can expect NATO leaders to discuss Manchester as an example of the threat of terrorism in Europe," Besch added. "What they can do, what they are doing but should perhaps invest more resources and personnel in, is engagement in unstable countries like Iraq, Afghanistan, or perhaps even Libya, through training of armed forces and police.
"They should also get much better at intelligence and information sharing, not just between NATO allies, but also between NATO and the EU."
Last year, NATO recognized cyber warfare as an official front, and is taking steps to deal with the shifting challenge of terrorism. But while counter-terrorism and spending were key issues, European allies want to ensure Trump does not divert attention away from Russia, according to Patricia Lewis, head of the International Security Department at Chatham House.
The key question was "how much does the US set the agenda for NATO and how do you prevent divergence between NATO's European partners and Canada and the US," Lewis told CNN.
Russia will be on the agenda and there were signs that "behind the scenes Trump is prepared to talk about things," Lewis said.
"Article 5 is the bottom line for NATO -- that is what NATO is all about. The one and only time Article 5 has ever been invoked was on 9/11; that was in the defense of the US," Lewis added.
"Smaller countries can all show that they are doing a lot in terms of contribution. More importantly, if NATO capability is something that can only be provided by the US, and the rest of NATO can work in partnership only with the US, then that is where the problem is."
Trump's "obsolete" comment, Lewis said, may have been him "trying to grab the attention of NATO states and to urge them to step up and say 'this is about security, don't expect the US to always be there whatever.'"