During the 1955 System, Japan’s political scene was divided primarily along a Left–Right axis delineated by defence policy. To this day, it remains difficult for the main Right-wing party the Liberal Democratic Party and the Centre-Left Democratic Party to cooperate on defence issues in spite of the fact that cooperation would be in the national interest. This chapter investigates the main reason for that lack of cooperation which essentially resides in the main Left-wing party’s internal divides.