It was recently revealed that Japan's Self-Defense Forces now designate China as a “hypothetical enemy”. This phrase has a controversial history that stretches back to the era of prewar militarism. In the 1930s, the Japanese military designated the US as a hypothetical enemy. After World War 2, this designation was identified as a reason for the militarists' view of war as inevitable. A strong taboo against labeling other countries as hypothetical enemies therefore emerged. But as the collective memory of war has waned, so has the hypothetical enemy taboo. The fact that the label is now attached to China by Japan's defense establishment does not bode well for Sino-Japanese relations.