On this day, May 16, 1867, Prince Nikola thanked the Serbian Prince Mihail for the help in the weapons he received for the upcoming battles with the Turks. On that occasion, five thousand rifles arrived from Serbia.

In 1866, the Serbian Prince Mihailo Obrenović and the Montenegrin Prince Nikola concluded an agreement on a joint struggle for liberation from the Turks and the unification of the Serbian lands. Prince Nikola undertook to abdicate the throne in favor of Prince Mihailo Obrenović in the event of the unification of the two states. The already mature stage of the Serbian-Montenegrin diplomatic rapprochement, centered on Duke Mirko and Prince Nikola from the Montenegrin side, i.e. Prince Mihailo on behalf of Serbia, will reach its peak with the signing of a secret agreement, which was supposed to be a precursor to the planned unification of the two states. The highest expression of the good state of Serbian-Montenegro relations was the signing of the long-prepared Agreement on Cooperation between Serbia and Montenegro, which can be considered one of the most significant documents from the history of the Serbian people in the second half of the 19th century. In the summer of 1866, Prince Nikola sent to Belgrade his closest associate and personal secretary, Archimandrite Nićifor Dučić, a prominent Serbian patriot and participant in the Herzegovina Uprising of 1861-62. years. The goal of his mission was to persuade Prince Mihailo to be godmother again to the second daughter of the Montenegrin prince. Mihailo gladly responded to the invitation. Since he was prevented from personally attending the baptism, he sent his emissary, Miloj Lešjanin, head of the department in the Serbian Ministry of Foreign Affairs. Believing that during his stay in Cetinje, Lešjanin should, in addition to being baptized, also perform a delicate political task, Prince Mihailo tasked him with preparing a draft agreement on cooperation between Serbia and Montenegro and offering it to Prince Nikola for signing. The purpose of this document was for Belgrade and Cetinje to undertake to jointly prepare, in political and military terms, for the fulfillment of two major goals: the expulsion of the Turks from the Serbian ethnic area and the unification of the Serbian people. These two goals have been in the minds of Serbs for decades, so the political elite in Belgrade and Cetinje felt obliged to fulfill the people's demands. That's why during Miloj Lešjanin's conversation with Prince Nikola and his associates, there was no dispute about general principles and national goals, but the dynastic issue, included in this document, was brought up again. Prince Nikola's father still insisted that Prince Mihailo agree to adopt his son, if he did not have children and an heir, and thus enable him to come to the Serbian throne. According to the instructions received in Belgrade, Lešjanin replied to Duke Mirko "that Prince Mihailo will recommend to the Serbian people to choose Prince Nikola as his successor in the event that he does not have a direct heir, but that he will not undertake to force the people to accept such a suggestion." Prince Nikola agreed to include in the contract his consent to renounce the Montenegrin throne in favor of Prince Mihailo under certain conditions. After that, the initialing of the contract began. It was initialed by Miloje Lešjanin and Duke Petar Vukotić and then signed by Prince Nikola na Cetinje and Prince Mihailo in Belgrade in September 1866. The text of the contract reads:

In the name of the holy and co-equal Trinity, His Eminence the Prince of Serbia and His Eminence the Prince of Montenegro, having before their eyes on the one hand their sacred duties, and on the other hand the deplorable state of the Serbian people in Turkey, and enthused by the same patriotic desire, to work for liberation in sincere communication and the unification of their people, they agreed to settle all related issues in this secret agreement, and to mark in advance the scope of their joint action...

Article 1.

Their Highnesses the Prince of Serbia and the Prince of Montenegro undertake and give each other their word that they will work in the greatest agreement and sincerity, to prepare an uprising against Turkey as soon as possible, and to free the entire Serbian people in Turkey from the Turkish yoke and unite them into one Serbian state .

Article 2.

If, with God's help, this enterprise is crowned with complete success, that is, the entire Serbian people in Turkey are freed and united in the future great Serbia, then even under that condition, the Prince of Montenegro solemnly promises that he will join and unite Montenegro with that great country. , recognizing Prince Mihailo as the ruler of the entire Serbian state.

Article 3.

If, by God's help, the enterprise which is before our eyes and which is set forth in the first article is crowned with success, then the Prince of Montenegro promises solemnly that he will join Montenegro to Serbia and recognize Prince Mihailo as the ruler of the entire Serbian state.

Article 4.

In such a case, appreciating the highly noble and patriotic sacrifices of His Royal Highness the Prince of Montenegro, His Royal Highness the Prince of Serbia most solemnly promises in his own name and in the name of Serbia, that His Royal Highness the Prince of Montenegro and his family will not only retain their princely dignity, but also his special concern is to create a morally and materially great position for the ruling prince of Montenegro in the new Serbian state.

Article 5.

In particular, the now ruling prince of Montenegro is guaranteed the rank of prince of the ruling family and precedence over all, after the ruler and his legitimate heir, if there were one. In addition, the prince of Montenegro is provided with an annual civil list of 20,000 ducats. In the same way, the material and moral position of all close relatives of His Highness the Prince of Montenegro will be taken into account.

Article 6.

If Montenegro were to merge with Serbia, His Majesty the Prince of Serbia promises that all those elders and officials, who would be in Montenegro at the time of the merger as elders and officials, will be considered the same as other Serbian elders and officials and that they will be paid and their material position is to treat them equally in everything.