September and began transferring units to Thrace; the Balkan League responded in kind. On 30 September, the League presented the Ottomans with a list of demands regarding the rights of its Christian population. The Ottoman Empire rebuffed the demands, recalled its ambassadors in Sofia, Belgrade and Athens and expelled the League's negotiators on 4 October, with the League declaring war against the Ottomans. Montenegro had already begun military operations on 25 September. Prelude The Army of Thessaly crossed into Ottoman territory in the early morning hours of 5 October, finding most border posts to be abandoned. The first major clashes took place the following day when the 1st and 2nd Greek Divisions attacked Elassona, resulting in an Ottoman withdrawal towards Sarantaporo. At 7 a.m. on 9 October, the Greek infantry opened the Battle of Sarantaporo. The 1st, 2nd and 3rd Divisions attacked the Ottoman main line frontally. In the meantime, the cavalry brigade, 4th and 5th Divisions conducted a flanking maneuver from the west with the intention of striking the rear of the Ottoman positions. Despite the position being perceived as impregnable by its defenders, the main body of the Greek forces managed to advance deep inside the pass, while auxiliary units broke through the Ottoman flanks. The Ottomans abandoned their defensive line during the night, fearing encirclement. At 4 p.m. on 10 October, the 4th Division marched into Servia, while the Greek cavalry entered Kozani unopposed the following day. After their defeat at Sarantaporo, the Ottomans augmented the remnants of Hasan Tahsin Pasha's force with fresh reinforcements and organized their main defensive line at Yenidje (Giannitsa). On 18 October, Crown Prince Constantine ordered the bulk of the Army of Thessaly to head towards Yenidje despite receiving conflicting intelligence reports regarding the disposition of the enemy troops. In the meantime, the 5th Greek Division under Dimitrios Matthaiopoulos, continued its advance across western Macedonia, aiming to reach the Kailaria (Ptolemaida)-Perdika area, where it was to await further orders. There, the division would either unite with the rest of the Army of Thessaly or capture Monastir (Bitola). After crossing the Kirli Derven pass, it reached Banitsa (Vevi) on 19 October. The 5th Greek Division continued its march through the Florina plain on 19 October, halting temporarily north of Kleidi Pass (Kirli Derven) after learning that the Ottomans were massing their troops at Florina, Armenochori and Neochori. The following day a Greek advanced