Original sentence: Bulgaria had initially been neutral in World War II, but opportunistically became (or was forced to become) an ally of Nazi Germany in 1941.Bulgaria internal reasons to invade Vardar Macedonia because of the pressure put on the government by the great amount of immigrants from Macedonia who wanted to come back to their home places and rejoin their families.Slavic Macedonians were considered  ethnic Bulgarians up to the Balkan wars, and the Macedonian language is still regarded by the Sofia government a Bulgarian dialect
Edited sentence: Bulgaria had initially been neutral in World War II, but opportunistically became an ally of Nazi Germany in 1941.Bulgaria invaded Vardar Macedonia for largely internal reasons: Bulgarian nationalists have always considered Slavic Macedonians to be ethnic Bulgarians, and the Macedonian language to be a Bulgarian dialect