A friend asked me: "Why do you believe that the US would actually follow through on an Article V both in the Trump Era and at any other point?" Good question. Part A is easy: I think I have written enough about believing that Trump would not follow through on the US commitment to defend its allies. Indeed, I have written many times that Trump's US would likely block consensus since there is no automaticity to the invoking of Article V.
The question is whether in the past or future the US would keep a semi-incredible commitment--to make great sacrifices, including potentially Chicago for Berlin, Seattle for Paris, etc? There is plenty of work on this as it has been studied repeatedly over the years. Because I am lazy, forgetful, and not subject to peer review (the joy of blogging), I will simply assert:
Huh? The US lost 57,000 lives and then some in Vietnam, a place it really did not care about, a place where it had no history, in large part because of a concern that failing to support an ally there would have ramifications elsewhere. While academics can debate whether reputation and credibility matter, the US under many presidents has acted as if it does. That the US got seriously involved in Bosnia in 1995 precisely because Clinton had made a promise to allies--25k troops to help get you out... which turned into 25k troops to quell the conflict and keep its allies in but under NATO, not the UN.
The design of tripwires--during the Cold War and now--were designed to create sufficient stakes to make it hard for a President not to act. During the Cold War, it was not just 200,000 soldiers, sailors, marines and aviators, but also their spouses and kids. Lots of hostages to make sure the US would respond AND to make the adversary and the allies believe the US would respond. These days, the tripwire is far thinner but still exists. Not families but still enough troops to make their deaths an act that would draw in the US.
There is one big difference between the Cold War and now, other than that whole Trump thing. During the Cold War, far more authority was delegated to the NATO commander--SACEUR (always an American) so that they could respond quickly. These days, SACEUR has much less authority to act before a decision is made at the North Atlantic Council--NATO's decision-making body. So, yes, Trump can prevent NATO from acting as whole. However, in a crisis, if there are German, British, and Canadian troops in harm's way, my guess is that many will respond with or without NATO orders. The Americans? Now we have doubts about them, and, yeah, it would be hell of a crisis in US civil-military relations as the troops would want to fire at the Russians attacking America's allies. How that gets resolved? Damned if I know.
So, sure, the promise was always a hard one to believe and a hard one to carry out, but the US did as much as it could to make it credible until ... it elected Donald Trump.