Nations don't have friends, they have interests.
It was in the interest of the NATO nations to rely on the US for defense, and it was in the interest of the US to provide that defense.
Trump has not only abjectly failed to recognise the second part of that, he has also made denial of it into a mainstream political opinion in the US electorate.
The US paid for the armed forces to protect the rest of NATO, and all she got in return was the geopolitical clout to dominate the entire world and to shape international commerce to the massive benefit of America.
Isolationism made the USA a footnote in world affairs, before her involvement in two world wars made her into a hegemonic superpower, and her involvement in the Cold War, with the willing cooperation of NATO, then made her the world's ONLY superpower.
And with typical simpletonism, Trump and his MAGA morons have looked at one side of that ledger, and utterly failed to see the other. NATO cost the US Trillions - but she got in return the entire world. It was the bargain of the (twentieth) century.
There's no way to rebuild that. It was a one-time offer, from a world broken by two world wars, and terrified of Soviet invasion of Europe.
Europe is now far more politically and economically united than it was during the Cold War. The USA were able to use their dominance to make the rest of NATO buy American weapons systems, that no individual European state could have afforded to develop for themselves. But the EU can afford this stuff, as a united economic bloc. And that ability to pay was a potential threat to the US.
Trump has turned that potential threat to US dominance, into an actual EU opportunity to take a slice of the cake back. Something that US Presidents since Eisenhower have worked hard to avoid, because it will cost America her world domination.
It's not coming back.
Like Brexit, the Trump Doctrine of an American exit from NATO is irreversable. The egg can't be unscrambled. There's no going back, no matter how awful the consequences. NATO was, above all, built on the rock-solid certainty that Article 5 meant unflinching US protection - up to and including the nuclear annihilation of the aggressor - for any member who was attacked. The slightest hint that the US might not hold up her end of that, is inevitably and irreversably fatal to the alliance - or at least to the US role as its dominant member.
Europe now has to defend herself. So, why would she allow a foreign power, such as the US, to maintain bases in her territory?
The US went it alone (well, with only Israel as an ally) in her attack on Iran - but it would have been a very different, and much more expensive, war had the US been denied the use of airfields in the UK, Germany, and Diego Garcia (the latter being a UK colonial posession). And it's not just European bases the US depends upon for power projection. US bases in Saudi Arabia, Oman, Bahrain, Kuwait, and the UAE exist because those nations were prepared to help the US to project world-dominating power, in exchange for US protection against any attacker.
America can bomb Iran, because the USAF can re-stock and refuel at bases around the world. Without those bases, her ability is sharply curtailed. Bombing Tehran is still possible directly from the Continental USA, but only with long and complex mid-air refuelling routes. If denied overflight rights by intervening nations, these become longer still (bombers from UK bases often had to fly around France and Spain during the recent campaign). And if any intervening nation was not only uncooperative, but actively opposed to such US action, the whole exercise becomes extremely hazardous.
She can use aircraft carriers to mitigate this to some extent, but you can't operate big bombers from aircraft carriers. And without friendly overseas resupply ports, deployments become long and painful when trying to project power tens of thousands of miles.
The US traded reliability for hegemony. Now that she has shredded her reliability, why would the world allow her to keep her hegemonic position?
There was always a "Yankees Go Home!" undercurrent in every nation where Americans have overseas military bases. Now the people who were opposing that anti-US sentiment, by pointing to the value Uncle Sam brought as a gaurantor of overwhelming military force against any invader, are no longer able to support US military presence with a straight face.
It will take a while for the longstanding defensive situation to be un-picked and rebuilt without US involvement; And that time will be a dangerous one. But it's endpoint (if WWIII doesn't intervene) will be a far stronger Europe, and a weakened and impoverished USA.
I suspect that the long term winners here will be Ukraine. They have the military technology and skills that will be needed for the next war, while the USA has only the technology and skills that were needed for the Cold War. The switch to exchanging EU cash for Ukrainian skills and technologies, rather than exchanging European military bases for American skills and technologies, will be a difficult transition, but massively beneficial to both EU and Ukraine.
The USA will be left with a huge debt, that once paid for a massive military (rendered obsolete by the loss of her overseas bases), which had allowed her to dominate the globe both economically and politically, but which now achieves nothing.
