The North was led by a communist regime under Kim ll Sung who was supported by the Soviets. The South was led by a nationalist Rhee who was supported by the U.S. Since both wanted to conquer the entire Korean Peninsula and both were aligned with different political ideologies, it wasn't too unreasonable that Kim decided to attack the South. As a result, America, who adopted a policy of containment under Truman, wanted to expel communist influence as much as possible, which led to America playing a role in mobilizing a UN force to support the South.
