What justification did the north koreans offer for their attack on south korea?

In the early 1950s, Kim Il-sung traveled to Moscow and Beijing to seek support for the impending war. The Soviet Union was heavily involved in the militarization of North Korea and its plans for an offensive against the south. Mao Tse Tung transferred 50,000 People's Liberation Army soldiers along with their armaments to North Korea. Months before the first attacks in the North, the CIA noted a huge military mobilization by the North Korean military, but thought it was only a "defensive measure" and concluded that an invasion was "unlikely."

Under the pretext of retaliating for alleged small incursions of southern soldiers on the border, the North Korean army crossed the 38th parallel border with heavy artillery protection on June 25, 1950. The North Koreans had stated that troops of the South Korean army, under the command of President Syngman Rhee, had crossed the border first and, therefore, they intended to arrest and execute Rhee. The armed forces of both countries had already attacked the border before the war on a small scale.

On June 27, facing a successful initial offensive by northern troops, South Korean President Rhee ordered the evacuation of the capital Seoul. Before fleeing, he ordered what became known as the Bodo Alley Massacres on June 28, where at least 100,000 suspected Communist sympathizers were executed without trial.