The Japanese military didn't reform in the early 1950s. When the US wrote Japan's modern constitution, they included a passage, Article 9, that says:

(1) Aspiring sincerely to an international peace based on justice and order, the Japanese people forever renounce war as a sovereign right of the nation and the threat or use of force as means of settling international disputes. (2) To accomplish the aim of the preceding paragraph, land, sea, and air forces, as well as other war potential, will never be maintained. The right of belligerency of the state will not be recognized.

Therefore, Japan is not allowed to have a standing army. However, the need for defense was apparent. So in 1950, still during the US occupation, Japan formed a police reserve group, which was designed to defend Japan in case of attack. There already were police in Japan, so this wasn't a huge difference between being a policeman and joining the reserves. They weren't exactly working with the US, except in the sense that the US was occupying Japan so was in charge of the country. But anyone working in a government job faced the same issue. Taking these jobs was part of Japan resuming self-governance and getting rid of the US occupation. And they were only for domestic defense, while the US was there for international defense, and was doing so in the Korean War at that point. The Japanese were quite alarmed at the dangers of communist countries attacking, so the US fighting communism so close to Japan's shores was seen rather positively in Japan, as fighting an enemy they were afraid of. This historical moment was characterized by a policy called the "reverse course", according to which the US occupational forces started focusing more on fighting communism and enlisting Japan as allies in that. As such, the US was less alarmed at the possibility of the Japanese having a more organized militia. So the police reserve group became more organized when the US ended the occupation in 1952. At this point, they weren't working for the US at all, the US occupation was gone, though military bases remained. But the Japanese forces were completely separate from the US military bases. And then in 1954, these domestic police became a bit more militarized as the Self-Defense Forces. So there was this gradual remilitarization but it never went too far, and it was for the most part pretty separate from the US. The majority of Japanese people were sick of war and supported Article 9. And so it has been for many years, but there has always been a movement, small at first but now much more prominent, that has wanted to get rid of Article 9.