Assuming the post shortly after the massive Anpo Protests against the US-Japan Security Treaty, Kosaka's most pressing task was to restore good relations with the United States.

Kosaka recalled, "In the immediate aftermath of the Security Treaty riots, repairing US-Japan relations was our single biggest concern.

"[7] To this end, Kosaka visited the United States several times and helped arrange a summit meeting between Prime Minister Ikeda and President John F. Kennedy in Washington D.C. in June 1961.

[7] In August 1966, Kosaka and Yoshimi Furui headed an eight-member LDP delegation to visit China.

[5] In 1968, Kosaka stated his desire to visit Mongolia to search for the viability of economic assistance towards the country.

[2] On 24 July 1972, Tanaka also appointed him as chairman of the newly founded Council for the normalization of Japan-China relations in the LDP.

[10][11] The task of the council that consisted of 312 members was to reach a consensus, since the pro-Taiwan and pro-Peking factions over the whole peace treaty issue emerged in the party.