South of Verdun, the Third Army was renamed Army of Lorraine and was to watch for a German offensive from Metz, which left the remainder of the Third Army free to concentrate on the offensive into Belgium. The French armies invaded Belgium with nine infantry corps but ten German corps and six reserve brigades of the 4th and 5th armies lay between Metz and the north of Luxembourg. The German 4th Army under Albrecht, Duke of Württemberg and 5th Army of Crown Prince Wilhelm had moved slower than the 1st, 2nd and 3rd armies and the French offensive towards them was reported on 21 August. The French armies had few maps and were unaware of the size of the German force opposite, as the Third Army brushed aside small German detachments. On 22 August in the Third army area, the V Corps attacked dug-in German troops at Longwy at in thick fog and heavy rain, with no artillery support.