On July 6, 1916, the Kingdom of Romania declared war on the German Empire and Austria-Hungary, following the initial success of the Brusilov Offensive (a major Russian offensive against the armies of the Central Powers on the Eastern Front). The Romanian armies entered Transylvania (then part of the Austro-Hungarian Empire), together with Russian forces. However, German forces under the command of General Erich von Falkenhayn stalled the attack in November 1916, and drove back the Romanian forces. At the same time, Austro-Hungarian and Ottoman forces invaded southern Romania, forcing the country into a two-front war. The Central Powers drove deep into Romania and conquered the southern part of the country (Wallachia, including Bucharest) by the end of 1916. The Romanian forces, led by Marshal Constantin Prezan, retreated into the north-east part of Romania (Moldavia). In the summer of 1917 however, Prezan, aided by Ion Antonescu, successfully defended the remaining unoccupied territories against German and Austro-Hungarian forces led by Field Marshal August von Mackensen. General Alexandru Averescu led the Second Army in victory at the Battle of Mărăști (July 22 to August 1, 1917), while Major Generals Constantin Cristescu and Eremia Grigorescu led the First Army in victory at the Battle of Mărășești (August 6 to September 8, 1917).