Just a reminder, since 1900 there have been several times America had to go to war to defend itself or democracy. 
At the end of the Declaration of Independence one reads..." And for the support of this Declaration, with a firm reliance on the protection of Divine Providence, we mutually pledge to each other our Lives, our Fortunes, and our sacred Honor".
If indeed Putin invades Ukraine, then the Baltic States, at some point America will be called to the same decision that England came to when Hitler rose to power. 
The question was posed, I gave my answer. NATO has a lot of members, including Poland. If we walk away from Poland we walk away from all hope that the larger war can be avoided, if indeed Poland were invaded. 
I suspect if you ask the question of a former Marine general rather than a former corporal, you would receive the same answer. 
http://www.nato.int/nato-welcome/index.html#
Hugh