and St. Ménéhould. The Fourth Army had withdrawn to Sermaize, west to the Marne at Vitry-le-François and then across the river to Sompons, against the 4th Army, which had advanced from Rethel, to Suippes and the west of Châlons. The new Ninth Army held a line from Mailly against the 3rd Army, which had advanced from Mézières, over the Vesle and the Marne west of Châlons. The 2nd Army had advanced from Marle on the Serre, across the Aisne and the Vesle, between Reims and Fismes to Montmort, north of the junction of the Ninth and Fifth Armies at Sézanne. The Fifth Army and the BEF had withdrawn south of the Oise, Serre, Aisne and Ourq, pursued by the 2nd Army on a line from Guise to Laon, Vailly and Dormans and by the 1st Army from Montdidier, towards Compiègne and then south-east towards Montmirail. The new French Sixth Army, linked with the left of the BEF, west of the Marne at Meaux, to Pontoise north of Paris. French garrisons were besieged at Strasbourg, Metz, Thionville, Longwy, Montmédy and Maubeuge. The Belgian army was invested at Antwerp in the National Redoubt and at Liège, fortress troops continued the defence of the forts. Eastern Front Austria-Hungary had declared war on Serbia on 28 July and on 1 August military operations began on the Polish border. Libau was bombarded by a German cruiser on 2 August and on 5 August Montenegro declared war on Austria-Hungary. On 6 August, Austria-Hungary declared war on Russia and Serbia declared war on Germany; war began between Montenegro and Germany on 8 August. The Battle of Stallupönen (17 August) caused a minor check to the Russian invasion of East Prussia and on 12 August, Britain and France declared war on Austria-Hungary, as Austrian forces crossed the Save and seized Shabatz. Next day, Austrian forces crossed the Drina and began the first invasion of Serbia. The Battle of Cer (Battle of the Jadar, began and the Battle of Gumbinnen in East Prussia took place from On 21 August, Austro-Hungarian forces withdrew from Serbia. The Battle of Tannenberg began in East Prussia and in the Battle of Galicia the First Battle of Kraśnik was fought in Poland from Shabatz was retaken by Serbian forces and the last Austrian troops retired across the Drina, ending the First Austrian Invasion of Serbia. The First Battle of Lemberg began in Galicia and