What political theory suggests that if one nation becomes communist, neighboring nations will follow?

The political theory that suggests if one nation becomes communist, neighboring nations will follow is the Domino Theory. This concept emerged during the Cold War, particularly in relation to Southeast Asia. It posits that the spread of communism in one country could trigger a chain reaction, resulting in communism taking hold in adjacent countries, much like a line of dominoes falling over.

This theory was used to justify U.S. involvement and intervention in various conflicts, most notably in Vietnam. The overarching idea is that the geopolitical landscape is interconnected, and changes in government or political ideology in one country can influence its neighbors significantly.

The other theories mentioned do not capture this specific idea as effectively. For instance, the Chain Reaction Theory might imply a series of events leading to an outcome but does not specifically focus on political ideology. The Contagion Effect suggests a more general spread of behaviors or ideas and does not pinpoint communism as the focus. Expansion Theory typically refers to broader concepts of territorial or ideological expansion, lacking the specificity regarding the sequence of political shifts that the Domino Theory encapsulates.