In June 1950 North Korea invaded South Korea. The U.S. ambassador wrote to the Soviet deputy foreign minister asking that the Soviets convince North Korea to withdraw. The Soviet Minister answered that Soviet had no involvement in Korea.
Aide-Memoire from the U.S. government delivered to the Soviet deputy foreign minister by the U.S. ambassador, June 27, 1950:
My Government has instructed me to call to your attention the fact that North Korean forces have crossed the 38th parallel and invaded the territory of the Republic of Korea in force at several points. The refusal of the Soviet Representative to attend the United Nations Security Council meeting on June 25, despite the clear threat to peace and the obligations of a Security Council member under the Charter, requires the Government of the United States to bring this matter directly to the attention of the Union of Soviet Socialist Republics. In view of the universally known fact of the close relations between the Union of the Soviet Socialist Republics and the North Korean regime, the United States Government asks assurance that the Union of Soviet Socialist Republics disavows responsibility for this unprovoked and unwarranted attack, and that it will use its influence with the North Korean authorities to withdraw their invading forces immediately.
The Soviet reply, June 29, 1950:
1. In accordance with facts verified by the Soviet Government, the events taking place in Korea were provoked by an attack by forces of the South Korean authorities on border regions of North Korea. Therefore the responsibility for these events rests upon the South Korean authorities and upon those who stand behind their back.
2. As is known, the Soviet Government withdrew its troops from Korea earlier than the Government of the United States and thereby confirmed its traditional principle of noninterference in the internal affairs of other states. And now as well the Soviet Government adheres to the principle of the impermissibility of interference of foreign powers in the internal affairs of Korea.
3. It is not true that the Soviet Government refused to participate in meetings of the Security Council. In spite of its full willingness, the Soviet Government has not been able to take part in the meetings of the Security Council inasmuch as, because of the position of the Government of the United States, China, a permanent member of the Security Council, has not been admitted to the Council, which has made it impossible for the Security Council to take decisions having legal force.