The North Atlantic Treaty Organization was one of the most successful and effective international institutions of the 20th century.

Constructed as a brilliant compromise between voluntary association and collective endeavour it succeeded beyond all expectations in keeping the world free from tyrannical communist encroachment by the then Soviet Union, and in the end presided at the collapse of the whole Soviet empire and the return of many captive nations to freedom.

But in this second decade of the 21st century NATO faces new and unforeseen challenges that threaten its very existence and purpose.

First and foremost of these, by a long way, is the fundamental question of U.S. engagement. From the very start of the alliance in 1949, mostly engineered by the Americans and the British, the U.S. was the bedrock and by far the largest contributor.

The pledge at the heart of NATO that an armed attack on any one of its 29 members (originally 12) should be considered an attack on all, and that all member states should be ready to assist with armed forces, (the famous Article Five) was a revolutionary pooling of sovereign power on a scale never before seen.

So why is this tower of solidarity now shaking? The first answer is in two words — Donald Trump. As officials from the other member nations prepare for the NATO summit on July 11-12 in Brussels apprehension prevails. What will the U.S. president do? What will he say when he attends?

He has already made all too clear his distaste for multilateral organizations and arrangements such as the European Union, the North American Free Trade Agreement, the World Trade Organization and the United Nations itself. Will he disrupt the NATO summit the way he disrupted the Group of Seven gathering in Quebec a few weeks ago, when he rubbished the agreed communique, insulted Canadian Prime Minister Justin Trudeau and for good measure called for the re-admittance to the group of Russian President Vladimir Putin, the arch rule-breaker who was expelled for his disregard for international law?

In one respect only there is a degree of European agreement with the Trump approach. Clearly the European partners should share more of the overall defense and security burden. The U.S. is carrying far more than its fair share. The totemic figure of 2 percent of GDP has been established as the lower threshold member states should aim for.

Of course this begs a string of question, such as what should actually be included in the 2 percent. Does it just mean spending on manpower, missiles, warships, planes, etc. Or does it include army pensions, weapons research, intelligence, anti-terrorist resources and a variety of other areas necessarily nowadays part of the defense of citizens in their daily lives?

But generally the Europeans should clearly do more. The United Kingdom ticks the 2 percent box, and so, oddly, does Greece. But Germany, with its understandable pacifist instincts after the horrors of the Nazi era, and its huge GDP, is never going to get near the target.

Trump does more than urge a burden shift. He actually links his pressure on the European allies with threats about trade, seeming to say that unless the Europeans pay more he will tighten protection against European exports to the United States. Automobiles are strongly in his sights, especially BMW and Mercedes.

Will he also meet Putin outside the summit somewhere? Probably yes, and probably in Helsinki. What he will say not even his closest officials seem to know.

NATO also faces other deep problems over and above the Trump doubts. High on the list is the question of Turkey, hitherto NATO’s staunchest member, with one of its largest military forces.

But the newly re-elected president, Recep Tayyip Erdogan, seems to be wobbling about his commitment to the West. He is said to be buying aircraft and missiles from the Russians that could knock down American planes. So whose side is he on? Turkey’s departure from NATO would be a colossal blow?

There are deeper questions still about the whole nature of effective defense and whether an old-fashioned military alliance like NATO can deliver the security and physical safety modern societies need. Cyber attacks can disrupt sovereign states on a major scale, causing fatalities if utilities and basic services are seriously damaged. Yet it may not be clear where they originate or which country is responsible. Could an attack against a member be on a scale justifying armed response? That is the dilemma NATO faces.

And what if the biggest modern threat to a citizen’s daily safety now comes not from armies and land or sea battles but from embedded terrorist cells, murderous jihadis or from home-grown, radicalized gun-happy individuals who may live right next door? What use are billions spent on tanks and missiles, on colossal aircraft carriers or ever more expensive military weaponry, in defending people against this sort of immediate threat?

Far more important may be intelligence and surveillance, counter-terrorism, undercover policing, border controls and even conflict prevention in fragile states. Or new and more subtle aid and development programs to go to the very roots and sources of international terror and crime intertwined.

It is true that one aspect of the old and simpler defense world of the past in which NATO was born persists — namely continued Russian aggression. But even that comes in novel and disorienting forms, with hybrid warfare, electronic bombardment, fake news, undercover political disruption, and sinister assassinations and poisonings.

Gone are the old simplicities of the Cold War era when the enemy was identifiable, the cause of freedom clear, the balance against nuclear Armageddon maintained and most (if not all) battles won convincingly on battlefields.

Even if Trump was not set to wreck the old kind of NATO, a new model of collective defense and security is now needed. The forthcoming summit will show clearly whether that fundamental new reality has been grasped.

David Howell is a Conservative politician, journalist and economic consultant. He is chairman of the House of Lords International Relations Committee.

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