Macron's justification for a European army is revealing:
"We have to protect ourselves with respect to China, Russia and even the United States of America," Macron said on French radio, according to the Wall Street Journal.
Yes, Russia is a military threat to the border areas of NATO and to the cohesion of NATO. But America is a treaty ally and no military threat (Besides, wasn't it just yesterday that America was accused of not being willing to defend Europe?) And while China is hardly a friend of Europe, it is in no position to be a military threat to Europe.
So Trump is missing the point by saying Macron was insulting America by saying we could be a threat (wow, the French being rude to Americans--what a shocking development).
Clearly, a continent-wide armed force under control of the European Union isn't being pursued for a military goal. If that was the case Macron would be urging European NATO states to contribute more to the existing and successful military alliance that has defended Europe and provided the means for Europe to wage their splendid little war over Libya in 2011.
The purpose for wanting a European army is to create a European super-state under control of the European Union, which is the first step to creating the imperial state that suppresses the sovereignty of the member states.
Remember, the basic foundation for the EU is that the proponents believe the nations of Europe are the cause of all the wars in Europe. So the nations must be stripped of military and political power. "Ever closer union," as the EU puts it, will eventually strip the member states of autonomy. For "peace." It is a pure coincidence, they think, that America has provided the most peace and democracy in European history since World War II.
A common currency didn't promote a European single identity to bolster such a proto-imperial first step. In fact the Euro has exacerbated divisions among the member states.
So now the engineers of the imperial project will emphasize building a common military that bypasses the American-dominated NATO. It will be less effective in defending Europe from military threats than NATO is, but that isn't the point.
And Macron may get that European army (read as "military") given that the appeal of a European military for each individual member state that will believe supporting the common army means somebody else can be tricked into carrying the burden of manning and maintaining that army.
And the French will of course believe that their sophisticated and nuanced grasp of politics means they will dominate that army. But don't you dare suggest France give up their nukes to the EU!
UPDATE: I've heard that the French claim to worry about Russia, China, and America was regarding cyber threats. And in my own defense the linked article did change the language from the quote I cited from an earlier version. But Macron did say he wanted a European army to avoid relying on America.
Mattis has strong feelings on what you should do if you want to defend Europe:
Asked whether he supported an EU military, Mattis told reporters: "We see NATO as the cornerstone for the protection of Europe in the security realm and we fully support nations doing more to carry the load."
Weakening NATO is bad for the defense of Europe. Period. And weakening NATO is what a European Union military would do. By design.