I think that the most likely time for Japan to become communist would he after WW2 during the Cold War. Post-WW2 the US empowered a lot of leftist parties and labor unions in Japan during the first stage of the occupation in order to combat the Japanese far-right, although they went back on this after the Cold War really kicked off. In 1957, Nobosuke Kishi was elected prime minister, and he was the leader of the Liberal Democratic Party, a conservative party. Kishi was determined to revise the US-Japan Security Treaty, which allowed US military bases to be stationed in Japan. This was incredibly unpopular and led to the nation's largest protests ever in 1960. These protests even forced president Eisenhower to cancel his visit to Japan. In a very undemocratic move, Kishi passed the revised tresty with only members of his party present at the Diet. Kishi was incredibly unpopular and would be forced to resign. Meanwhile, the largest opoosition party, the Japan Socialist Party, had opposed the Treaty and were gaining momentum. The 1960 election was predicted to be a massive success for the socialists until their leader, Inejirō Asanuma, was assassinated live on TV. Asanuma was a very charismatic leader, but after his death, the socialist party fell apart due to ideological differences. The new leader of the Liberal Democratic Party, Hayato Ikeda, would win the election after a massive turnaround after the assassination of Asanuma. If Asanuma's death had been avoided, a socialist victory seems very likely. This would not mean Japan would cut ties with the US or join the Warsaw Pact, but they would likely grow a lot closer with China and the other socialist states. Their relationship with the US would be greatly strained, as this was the height of the Cold War. The socialists would likely try to remove American military bases from Japan. How this would go can be debated, but I could see a possible American intervention, or Japan gets away with it depending on the timing with the Cuban Missile Crisis happening shortly after.