Since the end of the Cold War, NATO has been a military alliance of like-minded states with broadly similar cohering interests. In many respects, it was looking for a mission, with the result that peacemaking/nation-building missions came to be its main occupation, with many missions into various Balkan states as well as Afghanistan becoming the norm. Let's not mention Libya. Large-scale, peer-to-peer conflict (i.e. Russia), although nominally still the central mission, was definitely not a priority, given the state of European military budgets and procurement over the past 20 years. Sadly, it took until the 2022 Russian invasion of Ukraine, not the 2014 invasion, for European states to review their defence. Even then, it took a kick in the pants from Trump to concentrate minds.
Will the US and NATO part ways? Maybe. But in my recollection, European/EU defence has been much talked about for nearly 40 years without any serious steps being undertaken. The EU, by design, though dominated by France and Germany, has many conflicting interests, and finding the multinational coherence found in NATO (with the US) presents significant challenges. From personal experience, when the Europeans tried ops without a heavy US presence - Libya - it was a significant challenge. And that was working within the NATO construct. To my mind, what the US brings in military capability is scale, depth, breadth: compared to Europe, it is greater than the sum of its parts. And this brings the political weight and leadership that is difficult (though not impossible) in the EU. Thus, for example, a single American battlegroup deployed to the EFP brings a political-military weight that is really currently unmatchable. How will a European NATO/EU match this weight?
What next? Ironically, I think that the continued heavy US presence precludes a strong Europe. The cheapest option and the path of least resistance has been to let the US do the heavy lifting. That has to change, whilst we, IMHO, have to keep the US as a politically engaged partner. The recent NSS signalled that the US would be willing to maintain an interest in Europe, but its main effort (outwith N America) is East Asia. The challenge will be dealing with the Ugly American currently in the WH, whilst Europe (re-)builds a credible military capability and comes up with a future security framework that keeps America engaged and interested. Have you noticed some European nations quietly strengthening bilateral ties with the US? Reasonable hedging, IMO.
Russia? Yes, it's been in decline for 40 years. Doesn't mean it isn't dangerous or that it can't pose significant future problems for Europe. The bottom line IMO is that if Russia isn't within a European security framework, it will be within one with China; the latter is the current direction of travel, but not one that is in our interest. The challenge is to secure Ukraine's sovereignty and security, whilst weaning Russia from its current path. Dealing with a morally reprehensible Russia will be a Kissinger-esque challenge, a bad option or a worse option. At the moment, Trump seems to be filling a void: the worst European conflict since 1945, and it seems that Europe isn't even at the table. I know they have some leverage through Trump, but that's not the same.
Canada? Is this a moment to reflect on what our national security-defence situation is in, say, 2050? A root and branch review would inevitably put our long-term relationship with the US to the forefront, but presumably, a review of military commitments to Europe is also necessary. IIRC, Pierre Trudeau briefly considered withdrawal from NATO. Inevitably, finite resources entail hard decisions and a cost-benefit analysis on where our national interest lies would be of value. Interested in people's thoughts on a long-term (mid-21st C) horizon.