Donald Trump has made his first foreign tour as President of the United States and, like his gauche moments, his "America First" attitude may turn out to be an aberration in the country's diplomacy. But the world cannot count on it. He is not the first US President to complain that Nato allies are not carrying their weight in defence spending but he is the first to let that complaint call into question the country's pre-eminent position in the alliance.

Furthermore, his failure to publicly endorse Nato's central security guarantee - that an attack on any member would be treated as an attack on them all - will have made a profound impact in Moscow and may bring a fundamental geopolitical realignment that will be lasting. It is one thing to display no awareness of the alliance commitment in comments during an election campaign, it is quite another thing to take this attitude into formal talks with alliance partners after four months in office.

The reaction of Germany's Chancellor Angela Merkel suggests the Trump the world saw and heard in public on this tour was the Trump that European leaders met in private. He did not look comfortable or assured in their company. He appears not to be a natural leader at this level and probably has no particular wish to be. Merkel told an audience on Sunday, "The times in which we can fully count on others are somewhat over, as I have experienced in the past few days. All I can say," she continued, "is that we Europeans must really take our destiny into our own hands."

Merkel is facing an election in September and she was speaking at a campaign event in a Bavarian beer tent but she was serious. As Germans reportedly see it, Britain's decision to leave the European Union is more than an economic separation, it has strategic implications too. If Europeans see the Western Alliance fragmenting into three more independent components: the US, UK and the EU, it would not be an entirely unwelcome development for the world.

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The West does not have as much need for solidarity as it did during the Cold War. Vladimir Putin's Russia remains a destabilising threat to its neighbours but Putin's only wider ambition is to ensure Russia remains a player in world affairs. If the US, UK and EU each now has separate dealings with Russia, it may be for the better. If nothing else, it means Russia will not feel as threatened by Nato.

A world in which the US relinquishes a dominant diplomatic role, the UK can no longer support the US from inside the EU, and Europeans "take our destiny into our own hands" is one in which Germany must figure more prominently. An armed and more cohesive EU could incorporate France's nuclear weapons and Germany's economic strength, taking a position among the superpowers, the US, Russia and China.

If this four-power world eventuates it will be hard to reverse no matter how hard America's next President works to try to restore his country's pre-eminent global position. Trump's presidency may mean America will never be great again.