SEATO was an acronym that stood for Southeast Asia Treaty Organization. This was an organization established in 1955. It was essentially supposed to be a version of NATO for Southeast Asia. Let us look at the main purposes of this alliance.
In part, the alliance was meant to be a basic collective security alliance. What this means is that all of the members promised to help out if any one of the members were to be attacked by a foreign country. In this case, they all promised to help if there were an armed insurgency in a country as well. In such an organization, an attack on one country is to be treated as an attack on all of the countries. This is supposed to keep the peace by deterring would-be attackers.
SEATO’s main purpose was to contain communism. During the Cold War, the US was very concerned with preventing communism from spreading any further than it already had. The US feared that communism might spread out of China and into other countries of Southeast Asia. Therefore, it created this alliance to help prevent this from happening.
An alliance like SEATO is meant to prevent attacks on its members. SEATO, in specific, was meant to prevent the spread of communism in Southeast Asia.