How did president truman respond to north korea attacking south korea?

He responded by supporting and initiating intervention using the United States army even without the support of the congress. This is why the war later started getting called Truman's war since he didn't actually get support from the congress so they used it against him later during his career. He supported the intervention believing that the congress supported him unofficially.

It can provide a social code to protect each citizen's future economical earnings. and then obviously defending the country from invading countries.