In 1950, before the war broke out, he negotiated the first agreement on American military aid to Korea, worth $10 million at that time.

Later that year, in testimony to Congress, Muccio called for increased assistance to Seoul and warned that Communist forces were a growing threat north of the 38th parallel.

[7] After the North Korean invasion in June 1950, and the dispatch of U.S. army divisions to defend South Korea, Muccio informed the State Department that U.S. commanders had decided to fire on refugees approaching U.S. lines, for fear of enemy infiltrators.

His letter, dated July 26, 1950, warned of “repercussions in the United States from the effectuation of these decisions.” [8] On that same day U.S. troops began a three-day slaughter of South Korean refugees in what is known as the No Gun Ri massacre.

[9][10] Through the first two years of the war, before he returned to State Department duty in Washington, Muccio was a crucial liaison in exerting U.S. influence over the South Korean president, Syngman Rhee, helping set the stage for armistice negotiations.