Complex sentence: At around 15:00 on the same day, the Bulgarian advanced guard engaged the enemy, first achieving success on the left flank of the front, after the detachment of Captain Popov had seized the heights of Divan and Cherni Vrah.
Split into simpler sentences: At around 15:00 on the same day, the Bulgarian advanced guard engaged the enemy. The Bulgarians first achieved success on the left flank of the front after the detachment of Captain Popov had seized the heights of Divan and Cherni Vrah.