Despite this compromise, the Serbs and Greeks remained wary of the Bulgarians. In 1913 the Second Balkan War began, the Greeks, Montenegrins, Serbs, and Romanians joining forces with their previous enemy, the Turks, against their former ally, the Bulgarians. This rivalry had been fostered by both Austria and Russia. Eventually, the Bulgarians turned to the Russians for arbitration and finally signed a mutual defense treaty with Russia. When the Romanians crossed into Bulgaria, the Bulgarians--who were simultaneously fighting in Macedonia and were therefore weakened by fighting on two fronts--were forced to surrender. As a result of this loss, when the peace treaty of Bucharest was signed in August 1913 and Macedonia was partitioned between Greece and Serbia, Bulgaria managed to retain only a tiny fragment in the eastern sector. Macedonia, however, remained an issue for Bulgaria. In World War I Bulgaria succeeded in invading Macedonia. During the interwar period Macedonia was divided between Greece, Bulgaria, and Yugoslavia, Yugoslavia retaining the largest portion of the land. In the 1923-34 period Macedonian terrorism plagued the country and wreaked havoc on Bulgarian political and social life. During World War II the Bulgarians invaded both Greek Macedonia and Yugoslav Macedonia once again. Although the Macedonians themselves were divided in their sentiments between loyalties to Greeks, Yugoslavs, and Bulgarians, the land eventually reverted to Yugoslavia during World War II. As an issue, however, it still burns in the minds of the Bulgarians. The Macedonian question has been aptly referred to as "that eternal Balkan sore spot of rival nationalism." WORLD WAR I As was the case in the Balkan wars, Bulgaria's primary motivation for engagement in World War I was irredentism. Again the country was determined to regain the two lands that had escaped her grasp in the past: Macedonia and Thrace. Although Macedonia was prized for political and social reasons, Thrace represented a strategically more significant objective. In order to develop foreign trade, Bulgaria required an outlet to the sea; Thrace represented that outlet. The domestic situation in the country before World War I was mixed. Although Bulgaria's army had been demobilized at the end of the Second Balkan War (1913) and economic conditions were rapidly improving, the mood of the monarchy and the middle class was one of vindictiveness and retaliation against those countries that had stripped Bulgaria of its territories. The country became divided between those who wanted closer relations with Russia and the Triple Entente and those who preferred an alliance with the Central Powers. As the war neared, the struggle between these camps intensified.