The Battle of Mars-la-Tour (also known as the Battle of Vionville or Battle of Rezonville) was fought on 16 August 1870, during the Franco-Prussian War, near the village of Mars-La-Tour in northeast France. One Prussian corps, reinforced by two more later in the day, encountered the entire French Army of the Rhine in a meeting engagement and, surprisingly, forced the Army of the Rhine to retreat toward the fortress of Metz.

A cavalry patrol of the 1st Squadron of the 1st Hanoverian Dragoon Regiment No. 9, led by Rittmeister Oskar von Blumenthal, discovered that Marshal Franois Bazaine's 160,000-man Army of the Rhine was attempting to escape from Metz to join with French forces at Verdun. This intelligence prompted General Prince Friedrich Karl, commander of the Prussian Second Army, to order at 1900 on 15 August a grossly outnumbered group of 30,000 men of the advanced III Corps under General Constantin von Alvensleben to cut off the French line of retreat at Mars-la-Tour and Vionville.

At 0900 on 16 August, they engaged the French army near Vionville, east of Mars-la-Tour. III Corps routed the French 2nd Army Corps and captured Vionville at 1130, blocking any further escape attempts to the west. From 1200 to 1600, Alvensleben defeated all attempts by four French corps to dislodge his III Corps. Moving autonomously toward the sound of battle, the arrival of X Corps under General Konstantin Bernhard von Voigts-Rhetz to the west and that of IX Corps under General Albrecht Gustav von Manstein to the east, solidified the German position after 1600.

On 16 August, the French could have swept away the key Prussian defense and escaped. Alvensleben attacked the French advance guard, believing that it was the rearguard of the retreating Army of the Rhine. Despite his misjudgment, Alvensleben held off four French corps for seven hours. The aggression and skill of the Prussians prevailed over Bazaine's gross indecision. Prevented from retreating, the French inside Metz had no choice but to fight the Battle of Gravelotte on 18 August.

The Franco-Prussian War or Franco-German War, often referred to in France as the War of 1870, was a conflict between the Second French Empire and the North German Confederation led by the Kingdom of Prussia. Lasting from 19 July 1870 to 28 January 1871, the conflict was caused primarily by France's determination to reassert its dominant position in continental Europe, which appeared in question following the decisive Prussian victory over Austria in 1866. According to some historians, Prussian chancellor Otto von Bismarck deliberately provoked the French into declaring war on Prussia in order to induce four independent southern German states—Baden, Württemberg, Bavaria and Hesse-Darmstadt—to join the North German Confederation; other historians contend that Bismarck exploited the circumstances as they unfolded. All agree that Bismarck recognized the potential for new German alliances, given the situation as a whole.France mobilised its army on 15 July 1870, leading the North German Confederation to respond with its own mobilisation later that day. On 16 July 1870, the French parliament voted to declare war on Prussia; France invaded German territory on 2 August. The German coalition mobilised its troops much more effectively than the French and invaded northeastern France on 4 August. German forces were superior in numbers, training, and leadership and made more effective use of modern technology, particularly railways and artillery.

A series of swift Prussian and German victories in eastern France, culminating in the Siege of Metz and the Battle of Sedan, resulted in the French Emperor Napoleon III captured and the army of the Second Empire decisively defeated; a Government of National Defense was formed in Paris on 4 September and continued the war for another five months. German forces fought and defeated new French armies in northern France, then besieged Paris for over four months, before it fell on 28 January 1871, effectively ending the war.

In the waning days of the war, with German victory all but assured, the German states proclaimed their union as the German Empire under the Prussian king Wilhelm I and Chancellor Bismarck; with the notable exception of Austria, the vast majority of Germans were united under a nation-state for the first time in history. Following an armistice with France, the Treaty of Frankfurt was signed on 10 May 1871, giving Germany billions of francs in war indemnity, as well as most of Alsace and parts of Lorraine, which became the Imperial Territory of Alsace-Lorraine (Reichsland Elsaß-Lothringen).

The war had a lasting impact on Europe. By hastening the process of German unification, it significantly altered the balance of power on the continent; with the new German nation state supplanting France as the dominant European land power. Bismarck maintained great authority in international affairs for two decades, developing a reputation for adept and pragmatic diplomacy that raised Germany's global stature and influence. In France, it brought a final end to imperial rule and began the first lasting republican government. Resentment over France's defeat triggered a revolutionary uprising called the Paris Commune, which managed to seize and hold power for two months before its bloody suppression; the event would influence the politics and policies of the Third Republic.