The US pointing out that European NATO members underspend on defense goes back to JKF, and the reaction has always been to nod, smile, and ignore the issue. Very pointed warnings about this issue were given by the US Secretary of Defense in 2013:
https://www.atlanticcouncil.org/blogs/natosource/text-of-speech-by-robert-gates-on-the-future-of-nato/
The whole thing is worth a read, but here's an excerpt:
With respect to Europe, for the better part of six decades there has been relatively little doubt or debate in the United States about the value and necessity of the transatlantic alliance. The benefits of a Europe whole, prosperous and free after being twice devastated by wars requiring American intervention was self evident. Thus, for most of the Cold War U.S. governments could justify defense investments and costly forward bases that made up roughly 50 percent of all NATO military spending. But some two decades after the collapse of the Berlin Wall, the U.S. share of NATO defense spending has now risen to more than 75 percent – at a time when politically painful budget and benefit cuts are being considered at home.
The blunt reality is that there will be dwindling appetite and patience in the U.S. Congress – and in the American body politic writ large – to expend increasingly precious funds on behalf of nations that are apparently unwilling to devote the necessary resources or make the necessary changes to be serious and capable partners in their own defense. Nations apparently willing and eager for American taxpayers to assume the growing security burden left by reductions in European defense budgets.
Indeed, if current trends in the decline of European defense capabilities are not halted and reversed, Future U.S. political leaders– those for whom the Cold War was not the formative experience that it was for me – may not consider the return on America’s investment in NATO worth the cost.
Not pulling any punches, was he? As usual, this was ignored, and here we are, 13 years later, with the prediction coming true.
I don't think the average Brit appreciates the cumulative effect decades of defense cuts have had on HM's forces. I'm a U.S. Army veteran, and I served alongside BA forces in Operation Granby in Iraq. Back then, the BA had about 150k active duty soldiers. It's now 72k, the lowest number since the 18th century. Your navy has gone from over 100 warships to 25. Of those, the five Astute-class fast attack submarines are in the dock for repairs because of a lack of maintenance funds. That's right, none are seaworthy. https://nationalinterest.org/blog/buzz/uks-astute-class-submarines-totally-undetectable-at-sea-for-one-reason-ps-061126. Recall a few months ago when it took three weeks for the RN to deploy a single warship to the Med when the Iranians lobbed a missile at the RAF base on Crete.
As to all of the discussion about tanks, Russia has no desire to invade the UK. What they would absolutely love to do is get revenge on a country they view as a historical enemy and adversary by using missiles and drones to destroy high-value targets in the UK, like oil and gas infrastructure, ports, communications nodes, and airports. At present, the UK has very little to hit back with that would deter the Russians, other than its NATO membership. The UK has also become increasingly irrelevant in global security discussions because it has so little to contribute.
So I'm pleased that the topic of defense spending is being discussed, but I'm not hopeful anything meaningful will be done, as any attempts to moderate the growth in social spending have been politically untenable, and there's very little room to raise taxes or borrow more money from abroad.