Because the previous experience of "invincible" warfare gave the French extreme confidence, the French soldiers actually thought that Prussia was vulnerable, and crossed the Rhine in a majestic, high-spirited, and chaotic manner before the assembly was completed. Marshal McMahon's Strasbourg Corps on the South Road seized Wissenburg with only one division on August 4, and the 5th Army of the 3rd Army of the Prussian Crown Prince immediately counterattacked, and two other corps assisted, with 50,000 men against the French 6,000 men, the French army was raided, the division commander was killed, and almost the entire division was destroyed. The next day, 15 miles to the southwest, the Prussian 5th Army, which continued to advance, rammed into the positions of the French 6th Army, where 5 infantry divisions and 1 cavalry division of the French army were assembled, with a total of 42,000 infantry and 6,000 cavalry. Before the battle, the French army itself was in disarray: they had neither reconnaissance nor sentry posts, the 50,000-strong army had only 6,000 rations, and the French soldiers, who were concerned about their civil rights and welfare, complained that a large amount of food was finally delivered that morning, and the French soldiers were cooking, and the Prussian army arrived. The vanguard of the Prussian 5th Army entered the battle without pause, and soon the 3rd Army arrived and engaged in the battle, and McMahon's Strasbourg Army came to reinforce it, so that an encounter eventually turned into a major battle between the main southern armies of the two armies.

In this battle, the French soldiers showed great bravery, and the Prussian armies also fought hard one by one. The Prussian 11th Army infiltrated the rear of the French army according to a predetermined plan, and McMahon ordered a retreat for fear of being surrounded by the Prussian army. Two French cavalry brigades, acting as cover, charged the Prussian army's dense infantry formation, suffering three-quarters casualties. During these two days of fighting, the French lost a total of 25,000 men. The Prussian losses were heavier than those of the French, but strategically the Prussian army was victorious: Macmahon was forced to retreat from Strasbourg, ignoring the Bazin Army Group across the Vosges Mountains to the north, and retreated all the way west past Metz until the rear of Chalon. The French offensive was not smooth from the start.

To the west, the French army on the northern route, which had been massed in Metz, was similarly unsuccessful. On August 2, the main force of the French Rhine Army attacked Saarbrücken on the east bank of the Rhine, without even pre-battle reconnaissance, and the three companies of the Prussian defenders resisted for a while and then retreated, and the French newspapers boasted that "the heroic French soldiers smashed the three divisions of the Prussian army". Originally, Prussia's plan was for the 1st and 2nd armies to resist the main French forces and let the 3rd Army outflank the French army from the south. Hearing of the battle at Saarbrücken, the commander of the 1st Army, General Steinmetz, could not hold back and immediately waved his troops to meet the attack, so the Prussian 2nd Army also dispatched from Kaiserslautern to respond.

On August 4, the vanguard of the 1st Prussian Army encountered the French retreating from Saarbrücken, and the 14th Division of the Prussian Army took the lead in the battle without waiting for the orders of the superiors, and attacked the existing positions of the French army, suffering heavy casualties, but the troops near the Prussian army heard the sound of artillery according to tradition and immediately came to help, and although the French army was in a superior position, due to the surprise of the Prussian army and the lack of assistance from friendly neighbors, the commander of the 3rd Army, General Frossad, was worried that his position was too prominent, and decided to retreat at night. The Prussian army lost 4,500 men and the French lost 2,000 in this engagement. In terms of losses, the Prussian army was relatively large, but strategically, due to this engagement and the Battle of Wools in the south, the main force of the French Rhine Army was isolated and prominent, so that the French army's morale began to waver.

In the two battles of Saarbrücken and Wools, the Prussian army did not pursue, they had figured out the situation of the French army, and prepared to find the Rhine corps in Metz, France according to the established plan: the frontal 1st and 2nd armies drove back the French troops in front of them, so that the French retreated to the base in Metz. After the Southern 3rd Army defeated the French Strasbourg Corps, it was outflanking from the right flank of the French army, and a large army was gathered. The 180,000-strong army of the Rhine Army was the main force of the French field army, and now it is cowering in Metz and does not know where to go. Napoleon III realized at this time that the French army was too prominent and decisively ordered a retreat. At this time, the Prussian army was preparing to cross the Moselle on both sides of Metz and detour the two flanks of the French army, at this time the French high command was still in the dark, but knew that Metz was short of food, and suggested retreating, but at this time Napoleon III had already made up his mind to retreat westward, and his intention was to retreat first to Verdun, 40 miles to the west, and then 50 miles to the west, to join Chalon's MacMahon's corps.

Unlike the single-minded attack at the beginning of the war, Napoleon III's determination to retreat was so strong that he seemed to have changed a person, much to the surprise of his generals. Military observers attributed Napoleon III's performance to the fact that the heavy Prussian artillery fire inflicted on the French army affected the emperor's will to war.

The French had already decided to retreat, but the execution dragged on, delaying for three hours in the confusion, but they were able to retreat safely to the other side before the flood washed away the bridge over the Moselle. The Prussian army did not detect the French movement at first, but planned to cross the Moselle and continue westward to make a far-reaching encirclement. However, the brigade of Major General Golz of the Prussian 7th Army found that the French army had the intention of retreating, so it took the initiative to catch up and open fire, and then, General Mandolfel's 1st Army also entered the battle, and after an encounter, the Prussian army lost 8,000 men, and the French army lost another 6,500 men, although this time the French losses were smaller, but the process of French retreat was almost stopped by the Prussian army.

In Paris, the regent Empress Eugenie, enraged upon learning of the crushing defeat at the front and the retreat of the French army, repeatedly asked the high command for orders to stop the French retreat, and sent a telegram to Napoleon III asking her husband to halt the retreat, but Napoleon III simply ignored her angry accusations.

The next day, the almost frightened Emperor Napoleon III handed over the post of commander-in-chief (not only commanding the Rhine Corps, but also commanding the Chalonian Corps) to Marshal Bazin, and he led his retinue to break away from the troops and rush to Chalon, instructing Bazin to lead a large army to retreat from southeast to west to Chalonon according to the original plan. On the same day, the Prussian army, which had just intercepted the retreating French army, did not know that this was the total retreat of the 180,000-strong army of the Rhine Army, and thought that the French army was small, and continued to attack some of the French troops stranded east of Metz. General Bledau's cavalry brigade rushed into the dense formation of the French army, this charge, later known as the "Bledau Death Charge", the Prussian army simply broke through the two dense fire nets of the French army, but was surrounded by the French cavalry, and finally only half of the troops broke through the siege and killed back.

――――Dividing Line――――

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