What is a brief explanation of the attack made by the Bulgarians against the Serbs?

Arms and the Man by George Bernard Shaw is set during the Serbo-Bulgarian War. This was a war between Serbia and Bulgaria that started on 14 November 1885 and ended on 28 November 1885. More specifically, it recounts the events of the Battle of Slivnitsa that occurred on November 17–19, 1885 and represented a decisive victory for the Bulgarians. 
The battle is initially described by Catherine Petkoff in the very first scene of the play. She describes it as follows:

A cavalry charge—think of that! He defied our Russian commanders—acted without orders—led a charge on his own responsibility—headed it himself—was the first man to sweep through their guns.

A few pages later, we get an account of this same battle as seen through the eyes of a professional mercenary, who makes disparaging comments about the sheer foolishness of attacking a machine gun post with horses and swords. In his discussion of the charge the Swiss mercenary gives a very vivid description of the silliness of the charge and the accidental reason why it succeeded when it should have failed.