The domino theory refers to various theories stating that if one change occur, then this will cause a "domino effect"/chain reaction, leading to many more similar changes.
This is somewhat similar to the slippery slope concept, but the "domino effect" is seen as more automatic and inevitable by the domino theories, once the first change has occurred. Thus, this first change is extremely important to implement (if positive) or to prevent (if negative), justifying the use of the most extreme and violent measures.
During the Cold War, the domino theory was used as a propaganda justification for anti-Communist interventions such as the Vietnam War. The Communists won the war, but the predicted domino effect did not occur, with instead Communism soon collapsing worldwide.
After the Cold War, the domino theory has been used as a propaganda justification for wars such as the Iraq War, claiming that this would cause domino effect, with liberal democracy then spreading to other Muslim countries. This has been referred to as a "reverse domino theory" or a "democratic domino theory". See also democratic peace theory, Arab Spring, and Islamization.
Many genetics denialists and race denialists seem to believe in some kind of domino theory, apparently believing that if genetics is admitted to have even the smallest influence on even one politically sensitive (group) characteristic, then this will cause a domino effect leading to a Holocaust.