Original sentence: Skopje was liberated on 13 November 1944 by Yugoslav Partisan units of the Bulgarian People's Army (Bulgaria having switched sides in the war in September) aided by the Macedonian National Liberation Army
Edited sentence: Skopje was liberated on 13 November 1944 by Yugoslav Partisan units of the Macedonian National Liberation Army, together with units of the newly allied Bulgarian People's Army (Bulgaria having switched sides in the war in September)