President Trump is set to meet other Nato leaders for potentially fractious talks in Brussels on Wednesday with an eye toward pushing them to boost their defense budgets.
But as the meeting gets underway, we take a look at why this week's Nato summit matters:
More British troops are heading to Afghanistan
Theresa May is to commit 440 additional British troops to the Nato mission in Afghanistan.
They will begin deploying in August with a second contingent to follow in February taking the total UK military presence in the country to 1,100.
Around 16,000 troops from 39 Nato members and partner countries make up its Resolute Support Mission to train, advise and assist Afghan security forces and institutions.
Ahead of his arrival Donald Trump reminded Europeans that the US had spent "many times more" on their defence than any other alliance member.
Mrs May was keen to emphasise the UK was one of just five alliance members to meet the target of spending 2% of GDP on its armed forces.
Countries including France and German have already agreed to up their defence spending. Others are waiting to see what Mr Trump has to say.
A war of words
Aside from defence, Mr Trump and European figures have had bitter exchanges over trade in recent months.
On Tuesday, the President of the European Council, Donald Tusk, said the US "doesn't have and won't have a better ally than EU" and pointed out its members spend much more than Russia and as much as China on defence.
Mr Trump hit back at the European leader on Wednesday morning stating his country was "paying far too much" for decades and "not fair to the taxpayer".
Meanwhile the President of the European Commission, Jean-Claude Juncker, said "never has it made more sense to cooperate together and complement each other's work".
What Mr Trump could do
Following the acrimonious break-up of the G7 summit in Quebec in June, Mr Trump's comments on defence spending are likely to increase nervousness among European and Canadian leaders about his commitment to the alliance and the wider international order.
If he decides his demands are not being listened to the president could withdraw US troops from Europe.
There are currently around 65,000 American military personnel in Europe, down from several hundred thousand during the Cold War.
A withdrawal would weaken European defences against any Russian aggression.
Why does this matter now?
Vladimir Putin. The two are due to meet in Helsinki on July 16 following Mr Trump's visit to the UK at the end of the week.
Any moves to weaken Nato, which was formed in 1949 as a bulwark against Russia - then the USSR - would play right into Mr Putin's hands.