It was called the Revolutionary Insurgent Army of Ukraine led by Nestor Makhno as part of the Makhnovschina movement.

There was also the anarchist CNT-FAI which had an army of decentralized militias, collectively organized by Buenaventura Durruti during the Spanish Civil war of the 1930s.

During the Russian Revolution and early parts of the Russian Civil War, there were also a lot of anarchist militias and military units, most notably the Kronstadt sailors. The various groups never coalesced as a single army, and, therefore, were easily crushed by the Bolsheviks.

There was also the Korean People’s Association in Manchuria, which was an anarchist society of more than 2 million people in the late 20s/early 30s. They never had a whole army, but they did organize militias along anarchist principles.

The Zapatista movement in Chiapas, Mexico, founded in 1994 and still active today, is organized along decentralized principles and is closely associated with anarchism.

More recently, the YPJ and PKK operating in the Autonomous Administration of North and East Syria and the militias fighting the ongoing revolution in Myanmar are not entirely anarchist, but have strong principles of direct democracy at their core.

In all instances, the overall organization of the militaries were not entirely dissimilar to a traditional military. There were enlisted soldiers led by officers who gave orders that were expected to be followed. There was a higher level command structure which organized the army to distribute resources and coordinate strategy and tactics. The big difference, however, was that the leaders (officers) tended to all be elected democratically by the people they led and could be replaced/voted out democratically whenever the people who they led decided they needed to go…

There’s a common myth that anarchists are opposed to organization. Quite the opposite is true, in fact. Anarchists are all about organization. The thing we oppose is hierarchical power structures. Systems that place someone, anyone, above anyone else and say, “you must do what your superior tells you on threat of punishment” are inherently evil. But free associations are not. Rather than thinking of an officer in an anarchist militia/army as a leader whose commands must be followed or you’ll face steep punishment, think of them as a central coordinator. Their directives aren’t followed because you’ll be court marshalled or otherwise punished if you don’t obey. They’re followed because people at every level are included in the process and allowed to have their voices heard. Everyone has a degree of ownership and influence over the process. People follow directives because they understand where they’re coming from and why the decisions were made. Yet, if at any time someone decides they no longer want to take part, they have the option to just leave.