It was necessary to develop this success at once so as to widen the breach, and in the night of the 15th-16th the I Serbian Army (General Boyovich) came into action, while Voivod Stepanovich pushed forward the Jugoslav and Timok Divisions, which had been hitherto kept in reserve, beyond the lines occupied by the assaulting divisions; this he was able to do because General Franchet d’Espérey had insisted that they should be kept close at hand, although the Serbian Command wished to leave them much further back. At 15.45 hours the Timok Division passed beyond the outposts of the 17th Colonial Division, and as the trenches captured corresponded roughly to the Greco-Serb frontier, the Serbian troops made a demonstration of sympathy to the French, whose splendid victorious effort had enabled them to re-enter their fatherland. The same day the Yugoslav Division, after a long march, attacked on the Koziak (Hill 1550, north of the Vetrenik), captured Hills 1810 and 1825, lost the former in consequence of a counter-attack delivered by Bulgarian reinforcements which had just arrived, but recaptured it definitely. To the extreme left, the Franco-Serb Group and the 1st Group of Divisions came into action with the attack of the 16th Colonial Division on Zborsko, where the trenches to the west were captured, but the enemy managed to hold the village. In the night of the 16th-17th, on the left of the I Serbian Army, the advance of the 11th Colonial Division began, together with elements of the 3rd Greek Division; while to the right and left of the Cerna (east) a battalion of Senegalese occupied Staravina, the Greeks occupied Zovik. [Illustration: ENEMY ORDER OF BATTLE, SEPTEMBER 15, 1918. To face p. 227. ]