After Prince Alexander Battenberg informed the heads of state of the neighbouring Balkan countries of his accession to the Bulgarian throne, the latter appointed their diplomatic agents and consuls general to Bulgaria. First from the Balkan countries, on September 6, 1879, the Serbian diplomatic agent and consul general colonel Sava Gruic appeared before the Bulgarian Prince.

By decree of July 19, 1879, an agency of the Principality was established in Belgrade with the Serbian government. On August 3, 1879, Dimiter Kirovich was appointed diplomatic agent of the Principality of Bulgaria in Belgrade with a secretary and a translator. The Bulgarian diplomatic agent in Belgrade handed his letters of credence on September 27, 1879.

Very soon after the establishment of diplomatic relations between Serbia and Bulgaria the first interstate agreements were signed. On the following year, by a letter of June 23, 1880, the Bulgarian minister of foreign affairs Dragan Tsankov informed the Serbian diplomatic agent in Sofia that the government approved the Serbian draft for postal convention between Serbia and Bulgaria with some slight amendments. After brief consultations, on June 10, 1882, the minister of foreign affairs of Bulgaria Valkovich and colonel Sava Gruic – diplomatic agent and consul general of Serbia in Sofia, on behalf of their governments, “led by the desire to organize a secure, fast and regular correspondence and exchange of parcels between the two countries”, signed the first Postal Convention between Serbia and Bulgaria. It was ratified July 22, 1883. The same year a Special Telegraph Agreement between the two states was also endorsed. Although at this time Bulgaria and Montenegro did not have accredited diplomatic agents yet, the two governments made efforts to develop the relations between the two countries. Evidence of this is the letter sent by the Bulgarian foreign minister Konstantin Stoilov on February 7, 1883, to the Ministry of Foreign Affairs of Montenegro:

“I have the honour to bring to the knowledge of Your Excellency that His Highness, my august master, in his desire to give His Highness the Prince of Montenegro an evidence of his great will to maintain with him relations of good understanding and close friendship, and for the enhancement of the ties linking Bulgaria and Montenegro, intends to realize his long-standing plan to visit Their Highnesses in the coming month of April and personally present the assurance of his highest respects and expression of most sincere wishes for their happiness and the grandeur of the brave Montenegrin people, which has always kept up the flag of independence and of the interests of Southern Slavs…”

In response to this letter, the Montenegrin foreign minister informed on March 20, 1883, that the intentions of the Bulgarian Prince Alexander Battenberg are received with satisfaction by Prince Nikola and the people of Montenegro. In the talks concerning this visit, led by the mediation of the Russian ambassador in Istanbul, it was agreed Battenberg would arrive in Cetina on April 25.

On May 29, 1884, Serbia recalled its diplomatic agent in Sofia G. Simic. The reasons pointed out for this were the Serbian emigrants who had come to Bulgaria after an unsuccessful revolt of the radical party against King Milan in 1883 and the unsettled dispute over the possession of some estates near the village of Bregovo, east of the river Timok, known as the Bregovo question. The Bulgarian government duly set forth its stand on all disputable issues in a Memorandum of June 15, 1884, stating its wish a solution favourable for both countries to be found soon. The protection of Serbian interests in Bulgaria was taken over by the Greek diplomatic agency.

Next year, 1885 was marked by the most dramatic events in the history of the relations between the two states. After the Union of the Principality and Eastern Rumelia without the preliminary approval of the great powers, signatories of the Berlin Treaty, Bulgaria fell in a delicate political situation. It got even worse with the rumours of an eventual military action of Serbia against Bulgaria. On October 6, 1885, Prince Battenberg wrote a personal letter to the Serbian King Milan, assuring him of the sincere friendship of the Bulgarian people towards the Serbian people:

“Dear brother,
The news from Serbia are disturbing the population of Bulgaria. In the recent days they became so alarming that I decided to inform you about it… The Bulgarian people seems to believe all these rumours, aimed at severing the friendly and blood links between the two brotherly countries. Confident in the untruthfulness of these rumours and fully trusting the friendship that you have always manifested to me, I turn to you, dear friend, with the request to allow me to disprove all these alarming rumours and pour water on troubled waters in Bulgaria. The events in Rumelia have in no way changed the feelings of brotherly friendship, which the Bulgarian people has always cherished towards the Serbian people, and the union of the two Bulgarias hasn’t any hostile nature towards Serbia. I am only a spokesman of the feelings of every Bulgarian and I assure you of the friendship and brotherly love we all harbour for the Serbian people and its King…
Yours, Alexander”

On November 2, 1885, Serbia declared war on Bulgaria. After the signing of the Peace Treaty between Bulgaria and Serbia on February 19 (March 3) 1886 in Bucharest and its ratification on March 5, the diplomatic relations between the two neighbour countries quickly normalized.

The text of the Peace Treaty between Bulgaria and Serbia singed in Bucharest with the participation of the Sultan as suzerain is very brief and contains but one paragraph: “Between the Kingdom of Serbia and the Principality of Bulgaria peace is restored as of the day this Treaty is signed.” The signing of this treaty took place on the same date as the signing of the San Stefano Peace Treaty (1878), which ended the war between Russia and Turkey and brought the liberation of Bulgaria.

On October 13, 1886, an agreement was signed in Nis¤ for renewal of the Bulgarian-Serbian diplomatic relations and settlement of all pending questions between the two countries. The preamble read:

“After the government of His Majesty the King of Serbia responded positively to the proposal of the Bulgarian Principality’s government to renew the diplomatic relations between the two countries, and after it showed it was ready to renew the relations of friendship and neighbourliness, the following agreement was concluded between Mr. Dr. Stranski, Bulgarian diplomatic agent to Serbia, duly authorized by the Their Excellencies the regents of Bulgaria, on the one part, and His Excellency Mr. Dragutin Franasovic, Minister of Foreign Affairs of Serbia, acting on behalf of the royal government, on the other.”