The advance of the Greek Army faced fierce resistance which culminated in the 21-day Battle of Sakarya (August 23 – September 13, 1921). The Turkish defense positions were centred on series of heights, and the Greeks had to storm and occupy them. The Turks held certain hilltops and lost others, while some were lost and recaptured several times over. Yet the Turks had to conserve men, for the Greeks held the numerical advantage. The crucial moment came when the Greek army tried to take Haymana, south of Ankara, but the Turks held out. Greek advances into Anatolia had lengthened their lines of supply and communication and they were running out of ammunition. The ferocity of the battle exhausted both sides but the Greeks were the first to withdraw to their previous lines. The thunder of cannon was plainly heard in Ankara throughout the battle.