The Allied blockade eventually rendered the king's position untenable. In June 1917, when the British and French threatened to bombard Athens if Constantine remained, the king passed his crown to his second son Alexander and left Greece, although he did not formally abdicate. Venizelos was now free to throw full Greek support behind the Allied cause.

After Greece declared war on the Central Powers in July 1917, ten divisions of the Greek army fought with great valor along the Macedonian front. In 1918 they routed German and Bulgarian positions and pushed the front line northward. Germany and its allies soon capitulated, and Greek troops were among those who marched triumphantly into Constantinople. At the cost of splitting the nation, Venizelos had brought Greece into the war on the victorious side. To justify the cost of this result and heal the wounds caused by the National Schism, he returned to the Megali Idea.