UNION MAZE is a Star Army of Yamatai code name for a planned contingency operation for the scenario in which the Yamatai Star Empire enters a state of war with the Democratic Imperium of Nepleslia where the Star Army of Yamatai will conduct a full-scale planetary invasion of Planet Nepleslia, the universe's most heavily urbanized planet, with the goal of returning it to YSE control.

UNION refers to a rejoining of Yamatai and Nepleslia and MAZE is a reference to the complicated, difficult combat environment of hyper-urban Nepleslia. It is part of a secret series of other UNION contingency operations plans. The UNION MAZE plan is highly secret and known only by certain commanders in the Star Army of Yamatai who have received it on a need-to-know basis.

In YE 28, Nepleslia seceded from Yamatai fairly peacefully, and a peace agreement was made between the two nations. In YE 30, the Nepleslian government reformed itself and the treaty was made invalid, creating a rise in tensions between the two nations. In YE 32, a new treaty, the Nepleslian Treaty of YE 32, was approved by Yamatai but the Nepleslian senate was attacked by the NMX. Since then, the treaty has failed to be signed and the Nepleslian military has effectively controlled Nepleslia. Additionally, despite both nations facing the threat of the NMX, they repeatedly failed to develop any sort of cooperation in overcoming it.

Yamatai also has the following major grievances with Nepleslia:

1. Nepleslia does not recognize Nekovalkyrja as people
2. Nepleslia claimed Neo Kohana after it was granted its independence by the YSE
3. Nepleslia might ally with Lor, and Yamatai considers a war with Lor to be likely. See: Yamatai's Conflict With The Lorath

It was expected that relations will continue to worsen unless progress is made in the International Relations Conference of YE 36. With that in mind, the Yamataian military considered war a serious possibility and in YE 36 began actively planning for it. Second Fleet was the first major unit assigned to train for the operation.

Luckily, the IRC proved fruitful and resulted in a military alliance with Nepleslia and Lor, called the Defense Arrangement Treaty (of the) Alliance of Signatory States. This newfound cooperation with Nepleslia eliminated the need for UNION MAZE and allowed its assets to be freed up and directed toward another potential war scenario, QUIET RAGE.

UNION MAZE has the objective of establishing control of the Nepleslia star system. It is broken into several phases:

1. Space Superiority: Establish clear and safe space for Star Army operations.
2. Infrastructure dominance: Remove the ability of Nepleslia to effectively sustain military forces.
   1. Target war-related bases and factories
   2. Obliterate the Nepleslian military
3. Ground invasion
   1. Establish bases and secure areas
4. Long-term occupation and anti-insurgency campaign