What was the domino theory?

It speculated that if one country in a region came under the influence of Communism then the surrounding countries would follow the domino effect

The domino theory was a theory prominent from the 1950s to the 1980s, that speculated that if one country in a region came under the influence of communism, then the surrounding countries would follow in a domino effect. The domino theory was used by successive United States administrations during the Cold War to justify the need for American intervention around the world.