The US has long recognized that Putin is a thug, and that Russia's security would be threatened if Ukraine were allowed to join NATO. The US has, however, refused to consider this same security externality when it comes to Russia's aggressions towards Ukraine.
The annexation of Crimea by Russia came after Ukraine sought an association agreement with the European Union, not with NATO at that particular point, leading to Vladimir Putin citing security concerns about the expansion of NATO as a reason to invade.
The Ukraine conflict could have been prevented by engaging in diplomacy, and the deal that could end the war is for Ukraine to remain a neutral state. The question is, what is Putin willing to do in terms of evading other countries?
The decision by George W. Bush to expand NATO by adding Ukraine or Georgia would not enhance the security of the US because it would threaten the relationship between US and Russia due to Putin's nationalism, which is the source of his popularity in Russia.
The United States needs to be careful not to involve itself in every conflict around the world and should focus on preserving its strength to use it wisely. It is important to avoid historical precedent where conflicts and disputes led to wars and dragging the country into unnecessary conflicts.