[Goose]

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.

The US, with its 4-year cycle, may change by 180 degrees on any 4-year boundary.  In this cycle, there was a dramatic change

The simple fact is that ANY nation mat change by any number of degrees (including 180) anytime it becomes politically popular/necessary. It doesn't require a 4-year cycle. However, there are some "third rail" commitments that have existed for a very long time in the US. From Sept 30, 1800 (when the Treaty of Mortefontaine abrogated the indefinite term US military alliance with France) until February 28, 1942 (when the US signed the indefinite term Anglo-American Mutual Aid Agreement) the policy of no "entangling" alliances was followed. This policy was recommended by G. Washington and followed religiously until 1942. I make that to be 142 years. Even World War I did not force the US to waver. We fought as an "associated power" and signed no treaties.

FDR undoubtedly would have preferred to break precedent much earlier than he did. The attack by Japan upon US sovereign territory and our unavoidable entry into WW II undoubtedly was the "big enough" event to justify its necessity to even the most dedicated isolationists. The US was hardly "being fickle" with regard to this policy.

Similarly, when the US signed the NATO founding treaty on April 14, 1949 Article 5 had some "weasel words" included in it to allow the US to claim we were not making an open-ended commitment to military action. However, the clause stating an attack on one “shall be considered an attack against them all” is the one that the US presidents and their administrations have emphasized. All presidents from 1949 until the present have recognized that NATO would be an empty vessel if the US would not respond fully to an attack on other NATO nations. These administrations have repeatedly reiterated that commitment. During the Cold War the question always ask was "Would the US risk New York for Paris (or more commonly Bonn)". The answer was always an emphatic yes. Any doubt would have been very corrosive. For 76 years we were steadfast in that commitment. We are not "fickle". Our policy was changed by the realization that WW II was so disruptive that a firm commitment to military cooperation was essential. Subsequently the US realized that living with the continued threat of war in Europe was not tenable. Thus the 76 year commitment to a maximal Article 5 response. It is unclear (to me at least) why we now seem to be willing to accept the possibility of European wars as "not our problem".

[Goose]

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.

The US, with its 4-year cycle, may change by 180 degrees on any 4-year boundary.  In this cycle, there was a dramatic change

The simple fact is that ANY nation mat change by any number of degrees (including 180) anytime it becomes politically popular/necessary. It doesn't require a 4-year cycle. However, there are some "third rail" commitments that have existed for a very long time in the US. From Sept 30, 1800 (when the Treaty of Mortefontaine abrogated the indefinite term US military alliance with France) until February 28, 1942 (when the US signed the indefinite term Anglo-American Mutual Aid Agreement) the policy of no "entangling" alliances was followed. This policy was recommended by G. Washington and followed religiously until 1942. I make that to be 142 years. Even World War I did not force the US to waver. We fought as an "associated power" and signed no treaties.

FDR undoubtedly would have preferred to break precedent much earlier than he did. The attack by Japan upon US sovereign territory and our unavoidable entry into WW II undoubtedly was the "big enough" event to justify its necessity to even the most dedicated isolationists. The US was hardly "being fickle" with regard to this policy.

Similarly, when the US signed the NATO founding treaty on April 14, 1949 Article 5 had some "weasel words" included in it to allow the US to claim we were not making an open-ended commitment to military action. However, the clause stating an attack on one “shall be considered an attack against them all” is the one that the US presidents and their administrations have emphasized. All presidents from 1949 until the present have recognized that NATO would be an empty vessel if the US would not respond fully to an attack on other NATO nations. These administrations have repeatedly reiterated that commitment. During the Cold War the question always ask was "Would the US risk New York for Paris (or more commonly Bonn)". The answer was always an emphatic yes. Any doubt would have been very corrosive. For 76 years we were steadfast in that commitment. We are not "fickle". Our policy was changed by the realization that WW II was so disruptive that a firm commitment to military cooperation was essential. Subsequently the US realized that living with the continued threat of war in Europe was not tenable. Thus the 76 year commitment to a maximal Article 5 response. It is unclear (to me at least) why we now seem to be willing to accept the possibility of European wars as "not our problem".