General:
* Dissolved
* Abolished
* Broken up
* Dispersed
* Scattered
* Discontinued
* Demolished
* Terminated
* Shut down
* Ceased to exist
More specific:
* For groups: Disintegrated, split up, fragmented, unraveled
* For armies: Demobilized, discharged, mustered out
* For organizations: Liquidated, wound up, closed down
The best synonym to use will depend on the specific situation and the tone you want to convey.