News peg: US deploys another warship off the Korean peninsula.

      xxx

Some  historical context of the conflict between North Korea and South Korea from international law perspective can be gleaned from the following UN Security Council Resolutions in the 50s on armed clashes between the two: 

United Nations Security Council Resolution 82 (1950) of 25 June 1950
The Security Council,
Recalling the finding of the General Assembly in its resolution 293 (IV) of 21 October 1949 that the Government of the Republic of Korea is a lawfully established government having effective control and jurisdiction over that Part of Korea where the United Nations Temporary Commission on Korea was able to observe and consult and in which the great majority of the people of Korea reside; that this Government is based on elections which were a valid expression of the free will of the electorate of that part of Korea and which were observed by the Temporary Commission, and that this is the only such Government in Korea,
Mindful of the concern expressed by the General Assembly in its resolutions 195 (III) of 12 December 1948 and 293 (IV) of 21 October 1949 about the consequences which might follow unless Member States refrained from acts derogatory to the results sought to be achieved by the United Nations in bringing about the complete independence and unity of Korea; and the concern expressed that the situation described by the United Nations Commission on Korea in its report1 menaces the safety and well-being of the Republic of Korea and of the people of Korea and might lead to open military conflict there,
Noting with grave concern the armed attack on the Republic of Korea by forces from North Korea,
Determines that this action constitutes a breach of the peace; and
Calls for the immediate cessation of hostilities;
Calls upon the authorities in North Korea to withdraw forthwith their armed forces to the 38th parallel;
II
Requests the United Nations Commission on Korea:
(a) To communicate its fully considered recommendations on the situation with the least possible delay;
(b) To observe the withdrawal of North Korean forces to the 38th parallel;
(c) To keep the Security Council informed on the execution of this resolution:
III
Calls upon all Member States to render every assistance to the United Nations in the execution of this resolution and. to refrain from giving assistance to the North Korean authorities.
Adopted at the 473rd meeting by 9 votes to none, with 1 abstention (Yugoslavia)

 United Nations National Security Council Resolution 83 (1950) of 27 June 1950

The Security Council,
Having determined that the armed attack upon the Republic of Korea by forces from North Korea constitutes a breach of the peace,
Having called for an immediate cessation of hostilities,
Having called upon the authorities in North Korea to withdraw forthwith their armed forces to the 38th parallel,
Having noted from the report of the United Nations Commission on Korea1 that the authorities in North Korea have neither ceased hostilities nor withdrawn their armed forces to the 38th parallel, and that urgent military measures are required to restore international peace and security,
Having noted the appeal from the Republic of Korea to the United Nations for immediate and effective steps to secure peace and security,
Recommends that the Members of the United Nations furnish such assistance to the Republic of Korea as may be necessary to repel the armed attack and to restore international peace and security in the area.
Adopted at the 474th meeting by 7 votes to 1 (Yugoslavia)