At the NATO summit in Brussels, US President Donald Trump acted like a diplomatic bully. He arrived late, then shoved the Montenegrin prime minister aside to ensure his place in the front row. He turned what was planned as a short ceremonial address into a lengthy tirade against allies. Trump demanded imaginary money from NATO members to which he is not entitled. His speech was full of provocations and profanities. Many heads of state and governmental leaders were shocked. Even the American NATO delegation was not expecting that Trump would stage one of his feared speeches, where he goes off on his own tangent. America first? No, Trump first!

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What is alarming is that, unlike all his predecessors, the American president has refrained from specifically committing to the solidarity of the alliance and providing assistance in case of self-defense. Does this mean that he thinks that NATO, the Western military alliance, is superfluous? Doubts about Trump's attitude are understandable. Can allies in Eastern Europe now really rely on the US to stand with them if they are attacked by Russia? Misgivings about the US's commitment, as the most important protection force, would shake the whole alliance. This will please the Russian leadership surrounding Vladimir Putin. In the end, Trump has contributed to destabilizing the alliance. His stance towards Russia remained unclear at the NATO summit in Brussels.

Bernd Riegert is DW's Brussels correspondent

In his own world

An American president can, of course, attempt to steer the alliance in a certain direction. But this is only possible through sensible suggestions and by attempting to win people over with arguments. For the alliance is not a one-man-show, which would be difficult for someone with a character such as Donald Trump's to comprehend.

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The reaction from NATO, and from the German Chancellor, which Trump provoked, was very restrained. They may adhere to the NATO agreements to increase military spending, but remained publically silent on the US president's untenable rebukes. After this mini-summit in Brussels, the question must be asked, how can the populists in the White House be dealt with in future? On an operational level, NATO has already found an answer: simply ignore them. In practice, NATO will continue implementing its agreements with the assistance of American officials and diplomats on the ground. And this strange president can keep on living with his bizarre ideas in what appears to be his own world – reality is somewhat different.