In his speech commemorating the 100th anniversary of World War I on Sunday, French President Emmanuel Macron declared that self-interested nationalism marked "a betrayal of patriotism". The reverberations of his words were felt from across the Atlantic Ocean.

Because, as is his wont when he feels he has been slighted in some way, US President Donald Trump fired off a caustic series of early-morning tweets. On Tuesday, his salvo was against Macron, and his proposal that Europeans should defend Europe, as well as the bilateral trade deficit.

Since one of the tweets was remarkably insensitive given the occasion, it seems the earlier claims of camaraderie no longer apply.

The tweets have laid bare the widening divergences between Europe and the United States, which have become ever more apparent with each reaching of hands across the water. The traditional cross-Atlantic alliance is being tested more from within than by any challenge that had brought them together.

Given that, and Europe's history in the last century, it was no surprise to hear German Chancellor Angela Merkel warn against the rise of narrow nationalism, isolationism and the dire consequences that refusing communication and compromise might cause when speaking at the Paris Peace Forum on Sunday. Two days later, she too voiced clear support for the idea of a common European defense by telling a session of the European Parliament that Europe must "take our fate into our own hands".

Given their recollections of how a land of prosperity quickly became a land of self-brought death and destruction, European countries have every reason to call for cooperation and communication, and remain vigilant to the rise of nationalism or isolationism.

With the White House choosing to take offense at even minor or imagined pricks to its ego, and resorting to appeals to the red, white and blue at the slightest affront, it is giving an object lesson in how to lose friends.

The transatlantic bond between the US and Europe is showing signs of strain, and it is right for Europe to reflect on what it gains from that in a world that has fundamentally changed from how it was in the days that bond was forged.

Whether the US still has the will to be the world leader, as it emerged after the devastation of two world wars, is not really the question. The question now is, does it have any answers for the world's pressing challenges for it to be perceived as a leader by anyone else.