How was nationalism a divisive factor?

Answer 1

Well, some dates and a broader context would be helpful, but assuming you mean 19-20th century Europe, it was a huge factor in the great political upheavals of that time. While there are many examples, some of the bigger ones are the Balkan independence wars c 1830-1850, and the better known Balkan Wars c.1912. The Austro-Hungarian Empire had to deal with divisive nationalism during its history, and it was felt particulary during WWI, when, for example, the morning orders (for the military) had to be issued in over 10 languages. Shortly after the war, the empire was split into abt 10 different parts or so.