[GK3]

I wonder whether Russia could undertake a new invasion within the next 4 years. At that point, the leadership of the US has a good chance of changing and its commitment to NATO may change.(*)

Granted, the world has seen how fickle the US can be and needs to be better prepared for that fickleness. 
 
(*)This is assuming the US continues as it has for nearly 250 years.

Is the U. S. fickle, or is it just one guy? Have we been fickle in the past? We answered the bell multiple times in the last century.

As our British friends point out repeatedly, we "answered the bell" in WW I three years late and WW II two years late. No Zimmerman Telegram and no Pearl Harbor, would the US have answered at all? Open question. There has always been and still is today a very large percentage of the population that believes "Our detached and distant situation invites and enables us to pursue a different course".
 
When Washington said that, the US was indeed distant but not at all detached, as events in his own administration had amply demonstrated (Jay Treaty).  The US was unable to follow an "isolationist" policy from  1792 through 1815, as events clearly demonstrated. It was probably true that nobody could successfully invade us but it was amply demonstrated that we could be blockaded and our commerce destroyed. The US couldn't survive that. After 1815 the US was able to stay out of international affairs except when we decided to participate (Mexican war, Spanish War). The US greatly benefited from the Pax Britannia.  The US was allowed to build a great power economy without the associated costs. We got a "free ride" from Great Britain. By the early 1900s, cracks were forming in that system. The Pax Britannia was falling apart. After WW I, Britannia no longer ruled the waves. The policy of the US toward "neutrality" fully went out the window in 1942. That was "only" 150 years, and was somewhat of a fluke. Great Britain wanted peace and free trade to increase its national welfare. So did the US. Britain paid the price in the immense cost of her RN and the necessity to fight "small wars" every so often. That was to our great advantage. We paid very little for our own defense.

After WW II the US was the only "great power" nation that was not devastated by the war. We were going to have to take over Britain's role as they simply could no longer do it. After WW I the isolationist majority (at the time) wanted things to be as they were before. They couldn't be, as events demonstrated. NATO was founded by the US in order to 1) oppose Russian ambitions and 2) to force the European nations to not fight each other. The second part is often forgotten. By founding NATO the US made the implicit pact that we wouldn't be late to the party again. Technology had made our position not distant at all. The idea was that collective security works only if everybody subscribes to it. If one "great power" chooses instead to follow a policy of short-term self interest, it makes no sense for the other collective security participants to constrain themselves. It is a classical game theory situation.

In so far as the current administration follows a policy of short-term self interest, the commitment from other nations  is equally destroyed, if not immediately, very soon. They have their own isolationists to deal with. Even a reversal of policy by the US  (by say the next administration) has every likelihood of not "fixing" the broken social contract. The US was in  a unique position after WW II. We are not in that position today. It isn't impossible that trust and unity of purpose can be restored, but it will not be all that easy. Until now, the concept of NATO "failing" was not even considered. Today it is being considered at some level by all members and some nations may decide to go another way. France almost did under de Gaulle. It can happen, especially if the "big dog" is no longer a player. As with many things, it is much easier to destroy them that to create them.

Very True, but I do not see Russia being in that much better of a position, and perhaps in an even worse position.  They have by all accounts ground up the stored armor from yesteryear and is approaching the point that it will no longer be able to replace is losses in the Ukraine.
https://understandingwar.org/backgrounde...per%20year.
If I were the Ukraine, I would find a way to hit that single tank factory with a drone strike. 
Common sense would seem to indicate that Russia is far from able to launch an attack into NATO at the current time. This would seem to give European NATO countries some time to get their act together and time for us to decide what we as a country want to do.  
I believe that the Western European NATO countries will act and spend more money on their defense. 
Whatever Putin may want to do, he still has to deal with the small population, limited industrial ability to produce modern weapons. and a group of individuals who probably do not want to see their assets blown up and thrown away to satisfy his ego. At the same time Russia has the one thing that has saved it since Napoleon and that is its vast size and miserable Winters. Put all your factories East of the Urals and no one will ever touch them.  Even today I am not sure any cruise missile can reach those factories. 
You also cannot avoid the fact that our military has downsized so far it would be seriously challenged to fight even a one front conflict.

[GK3]

I wonder whether Russia could undertake a new invasion within the next 4 years. At that point, the leadership of the US has a good chance of changing and its commitment to NATO may change.(*)

Granted, the world has seen how fickle the US can be and needs to be better prepared for that fickleness. 
 
(*)This is assuming the US continues as it has for nearly 250 years.

Is the U. S. fickle, or is it just one guy? Have we been fickle in the past? We answered the bell multiple times in the last century.

As our British friends point out repeatedly, we "answered the bell" in WW I three years late and WW II two years late. No Zimmerman Telegram and no Pearl Harbor, would the US have answered at all? Open question. There has always been and still is today a very large percentage of the population that believes "Our detached and distant situation invites and enables us to pursue a different course".
 
When Washington said that, the US was indeed distant but not at all detached, as events in his own administration had amply demonstrated (Jay Treaty).  The US was unable to follow an "isolationist" policy from  1792 through 1815, as events clearly demonstrated. It was probably true that nobody could successfully invade us but it was amply demonstrated that we could be blockaded and our commerce destroyed. The US couldn't survive that. After 1815 the US was able to stay out of international affairs except when we decided to participate (Mexican war, Spanish War). The US greatly benefited from the Pax Britannia.  The US was allowed to build a great power economy without the associated costs. We got a "free ride" from Great Britain. By the early 1900s, cracks were forming in that system. The Pax Britannia was falling apart. After WW I, Britannia no longer ruled the waves. The policy of the US toward "neutrality" fully went out the window in 1942. That was "only" 150 years, and was somewhat of a fluke. Great Britain wanted peace and free trade to increase its national welfare. So did the US. Britain paid the price in the immense cost of her RN and the necessity to fight "small wars" every so often. That was to our great advantage. We paid very little for our own defense.

After WW II the US was the only "great power" nation that was not devastated by the war. We were going to have to take over Britain's role as they simply could no longer do it. After WW I the isolationist majority (at the time) wanted things to be as they were before. They couldn't be, as events demonstrated. NATO was founded by the US in order to 1) oppose Russian ambitions and 2) to force the European nations to not fight each other. The second part is often forgotten. By founding NATO the US made the implicit pact that we wouldn't be late to the party again. Technology had made our position not distant at all. The idea was that collective security works only if everybody subscribes to it. If one "great power" chooses instead to follow a policy of short-term self interest, it makes no sense for the other collective security participants to constrain themselves. It is a classical game theory situation.

In so far as the current administration follows a policy of short-term self interest, the commitment from other nations  is equally destroyed, if not immediately, very soon. They have their own isolationists to deal with. Even a reversal of policy by the US  (by say the next administration) has every likelihood of not "fixing" the broken social contract. The US was in  a unique position after WW II. We are not in that position today. It isn't impossible that trust and unity of purpose can be restored, but it will not be all that easy. Until now, the concept of NATO "failing" was not even considered. Today it is being considered at some level by all members and some nations may decide to go another way. France almost did under de Gaulle. It can happen, especially if the "big dog" is no longer a player. As with many things, it is much easier to destroy them that to create them.

Very True, but I do not see Russia being in that much better of a position, and perhaps in an even worse position.  They have by all accounts ground up the stored armor from yesteryear and is approaching the point that it will no longer be able to replace is losses in the Ukraine.
https://understandingwar.org/backgrounde...per%20year.
If I were the Ukraine, I would find a way to hit that single tank factory with a drone strike. 
Common sense would seem to indicate that Russia is far from able to launch an attack into NATO at the current time. This would seem to give European NATO countries some time to get their act together and time for us to decide what we as a country want to do.  
I believe that the Western European NATO countries will act and spend more money on their defense. 
Whatever Putin may want to do, he still has to deal with the small population, limited industrial ability to produce modern weapons. and a group of individuals who probably do not want to see their assets blown up and thrown away to satisfy his ego. At the same time Russia has the one thing that has saved it since Napoleon and that is its vast size and miserable Winters. Put all your factories East of the Urals and no one will ever touch them.  Even today I am not sure any cruise missile can reach those factories. 
You also cannot avoid the fact that our military has downsized so far it would be seriously challenged to fight even a one front conflict.