I can’t think of an effective diplomatic strategy for the U.S./Ukraine camp to have used against Putin’s invoking of Article 51. As I understand it, Article 51 action basically is this: “I am afraid of you so I am going to attack and neutralize you with my superior force and moral right.” Was that a legitimate justification for Putin to invade Ukraine with a large military force?

Isn’t that what George Zimmerman used as a justification for killing Trayvon Martin? The ‘Stand Your Ground’ rationale for being an aggressor? Probably Trayvon should have been more diplomatic, but the same question applies - what sort of diplomacy would have worked with George?

You never change things by fighting the existing reality.
To change something, build a new model that makes the old model obsolete.
R. Buckminster Fuller