The clearer the threat is, the more vital the alliance is. However, with the collapse of the Soviet Union and the end of the Cold War, the US-Japan alliance lost its common enemy.
Combined with the economic conflicts between Japan and the US, the US was compelled to find a specific threat for the US-Japan alliance as soon as possible.
By doing so, it could end the belief in Japan that an alliance with the US was no longer needed after the Cold War, and make the Japanese feel that they were facing a threat that they could handle alone.
In deliberately exaggerating the severity of the threat posed by North Korea, the US convinced the Japanese that the security provided by the alliance against common threat was greater than the costs Japan had to pay for the alliance.