When British forces came close to capturing the Dardanelles and Istanbul in the spring of 1915, Bulgaria considered joining the Entente, but Britain, France and Russia were not willing to take territory away from their allies, Romania, Serbia and Greece. On the other hand, Germany promised to give Bulgaria the original borders it had had after the Treaty of San Stefano. Bulgaria signed an alliance with Germany on September 6, 1915, mobilized its forces, and declared war on Serbia on October 14. Britain, France and Italy, allies of Serbia, responded by declaring war on Bulgaria.