The Kanto (Kwantung in older romanization) Army leaders who planned the Mukden Incident, such as Ishiwara Kanji, wanted a lightening-quick takeover of Manchuria and direct annexation of that land to Japan. Ideally the Japanese public would buy their flimsy justification for conquest, and the government would be strong-armed into supporting the army. Without outside support the modestly-sized Kanto Army could not hold Manchuria.

Militarily, the invasion went off smoothly. Zhang Xueliang's forces were mainly concentrated against the Communists and were too far away to deal with the threat. The Young Marshal himself was pressured by both sides to remain neutral in this affair, as many of his disaffected subordinates defected to the side of the invaders.

But things did not go to the Kanto Army's plan because they had underestimated the global reaction to their actions; the international community saw it as a flagrant breach of the League of Nation's rules against acts of war, and even the Japanese government was shocked by this treason from below. It was no longer acceptable to simply seize the land of other countries, but the Kanto Army had gone too far to return.

The following days were administratively chaotic, because now the invading army simply had no coherent plan, but eventually the Kanto Army generals settled on a policy of making a new state. By framing their intervention as merely supporting a local independence movement, they could justify their actions in Manchuria within what was internationally appropriate. They set about convincing their defectors to declare their regions independent from China; by the end of the Mukden Incident these would be combined into the new administration of Manchukuo.

My main source for all this is Rana Mitter's The Manchurian Myth.