How did the Wilsonian ideal of national self-determination become a justificiation of Communist-inspired wars in the 20th century?
Wilson proposed the idea that countries should be able to determine their own forms of government and their own government. This extension of government for the people, by the people, also mirrors the ideas of popular sovereignty of the 1800s, during which states were able to vote on issues of entering as a slave state, or entering as a state at all.
The relationship to Communism is that 20th century American leaders did not view Communism as a form of government that allowed people to choose their own government. In fact, Communism was viewed as a form of dictatorship, not as a free government. Because of this, Communists were viewed as revolutionaries "taking over" a country, not running for office in free elections. Even if a country voted for Communist leadership, it was of the American opinion that such votes were coerced and therefore invalid. Therefore, warring against Communist countries to establish free elections was seen as an application of the support for self determination.