At the beginning of the conflict, the Serbian army was poorly trained and ill-equipped, unlike the troops of the Ottoman Empire. The offensive objectives the Serbian army sought to accomplish were overly ambitious for such a force, and they suffered a number of defeats that resulted from poor planning and chronically being spread too thin. This allowed Ottoman forces to repel the initial attacks of the Serbian army and drive them back. During the autumn of 1876, the Ottoman Empire continued their successful offensive which culminated in a victory on the heights above Đunis. During the second conflict, between 13 December 1877 and 5 February 1878, Serbian troops regrouped with help from Imperial Russia, who fought their own Russo-Turkish War. The Serbs formed five corps and attacked Ottoman troops to the south, taking the cities of Niš, Pirot, Leskovac and Vranje one after another. The war coincided with the Bulgarian uprising, the Montenegrin–Ottoman War and the Russo-Turkish War, which together are known as the Great Eastern Crisis of the Ottoman Empire.