After WW2, Europe, Germany and especially France, tried to create the precursor of the European Union with a defense component. There should have been a common European army with European nuclear weapons a common command structure and soldiers from all members. When this didn't work out, NATO was intended as a European defensive alliance, without the US at first.

Why didn't those two things happen? Because of US influence: The US actively prevented any kind of military unification e.g. by preventing West Germany from going in that direction (as the occupier they could do so) and by alienating France and bribing and distracting the UK (by sharing plans for weapons such as nukes and submarines). When the NATO idea came up, the US offered their membership, but with a twist: NATO troops would always be under USian command. At every opportunity, the US made themselves indispensible for European defense and prevented any kinds of European political initiatives in that direction. That is why France rejected the NATO command structure for quite some time and was only "half a member".

This "arragement" had a downside of increased dependence and an upside of decreased spending and responsibility for the Europeans, and they got used to it. For the promise of US protection, Europe would give up its independence in that regard. Europe would be dependent on US-controlled military infrastructure such as AWACS, SOSUS, PATRIOT, and satellite coms and surveilance. Europe would be dependent on US-delivered weapons and weapons components. In many suppossedly national/european military hardware, there are some indispensable USian components like radars, engines or GPS, which is a double advantage for the US: they could sell something and have a means to stop any kind of use they dislike, either by withholding spare parts and support or by builtin remote shutdown switches. Europe participated in the occasional war on behalf of the US like Iraq or Afghanistan, and the US cleaned up some European messes like Yugoslavia.

It all seemed like a dependence that was one-sided, but still somewhat beneficial to the Europeans.

Now, the US are walking back on their promises and are actively breaking their side of the deal by e.g. questioning their future adherence to the mutual defense obligation in NATO article 5. This is a much bigger deal than it sounds at first: When you read the game theory of it, promises of mutual defense must be unwavering, there must not be any doubt in the mind of a potential enemy. If the rockets are flying, a red button must be pressed, no questions asked. Any kind of doubt will be exploited by an enemy, as can be seen by current Russian propaganda threatening Europe. In former times, the US would be first to condemn such threats and issue counter-threats. Nowadays: silence.

This means that the US effectively ended NATO. Mutual defense obligations and promises of defense from the US are now worthless. Of course nobody is cheery about that. A European common defense is years and billions (long scale) away. Of course nobody is cheery about that. After the Europeans dutifully obliged the US and accepted their declaration of article 5 after 9/11 (the only occasion where article 5 was ever invoked), the US now declare their ingratitude and unwillingness to ever reciprocate. Of course nobody is cheery about that.