The summit in The Hague involved an informal dinner on Tuesday and one working session on Wednesday morning. A very short summit statement has been drafted to ensure the meeting is not derailed by fights over details and wording

Founded in 1949, the North Atlantic Treaty Organization was formed by 12 nations to counter the threat to security in Europe posed by the Soviet Union during the Cold War, notably via a strong US presence on the continent

NATO`s ranks have grown to 32 countries since the Washington Treaty was signed 75 years ago. Sweden joined last year, worried by an increasingly aggressive Russia

NATO`s collective security guarantee, Article 5 of the treaty, underpins its credibility

It`s a political commitment by all countries to come to the aid of any member whose sovereignty or territory might be under attack. Trump has suggested he is committed to that pledge, but he has also sowed doubt about his intentions. He has said the US intends to remain a member of the alliance

Asked again on Tuesday whether he would abide by NATO`s security guarantee, Trump said: "There are numerous definitions of Article 5, you know that, right? But I`m committed to being their friend." He added only that he is "committed to life and safety"

The United States is NATO`s most powerful member. It spends much more on defence than any other ally and far outweighs its partners in terms of military muscle

NATO`s day-to-day work is led by Mark Rutte, a former Dutch prime minister. As its top civilian official, he chairs almost weekly meetings of ambassadors in the North Atlantic Council at its Brussels headquarters. He chairs other "NACs" at ministerial and leader levels. Rutte runs NATO headquarters, trying to foster consensus and speak on behalf of all members

NATO`s military headquarters is based nearby in Mons, Belgium. It is always run by a top US officer