If I understand correctly, after a decisive defeat at Kalkhin Gol, the Japanese military decided to attack the pacific rather than continue in China.
Why?
Was the loss that devastating? Were they trying to not anger the Soviets anymore? If so, why didn't move their efforts south?
It is more so that they absolutely needed the Southern Resource Area on the Dutch East Indies and all.
Without that everything grinds to a halt. And the continuing war in China was causing those resources to be cut off. And it was feared that the US would fight to aid in their defense. Thus it was optimal to begin the war on Japan's terms and in conjunction with an articulated strategy. While the Army continued to fight and commit resources to the mainland regardless of Navy plans right up to late 1944 when it became clear those men would be needed in the home islands.
Additionally the Army/Navy conflict was not inconsequential. China was an Army show and the Navy was increasingly resentful of lost glory and prestige after basking for years in Tsushima.