Military strategy relied largely on surprise, as Bulgaria had moved most of its troops near the border with the Ottoman Empire, in the southeast. As it happened, the Ottomans did not intervene and the Serbian army's advance was stopped after the Battle of Slivnitsa. The main body of the Bulgarian army travelled from the Ottoman border in the southeast to the Serbian border in the northwest to defend the capital, Sofia. After the defensive battles at Slivnitsa and Vidin, Bulgaria began an offensive that took the city of Pirot. At this point the Austro-Hungarian Empire stepped in, threatening to join the war on Serbia's side if Bulgarian troops did not retreat. Fighting lasted for only 14 days, from 14–28 November. A peace treaty was signed in Bucharest on 19 February 1886. No territorial changes were made to either country, but Bulgarian unification was recognised by the Great Powers.