Between 26 and 30 August 1914, the first major battle of the war on the Eastern Front was fought near Tannenberg in East Prussia. Opposite were the German forces led by General Paul von Hindenburg and his Chief of Staff, Erich Ludendorff. On the other side, Russian troops advanced under the command of General Yakov Zhilinskij, commander in chief of the North-Western Front, who commanded two armies. The 1st Army, led by General Pavel Karlovich von Rennenkampf, moved from the Niemen River with four army corps. The 2nd, led by General Alexander Vasilevich Samsonov, attacked from the Vistula and Narew, south of the Masurian Lakes, operating with five army corps. General Zhilinskij, commander of the Russian forces, has signed a military agreement with France that commits the Russian Tsar to deploy 800,000 men within 15 days of mobilization. For the weak organizational and logistical structure of the Russian army, this is too great an effort that unfortunately for them is accompanied by another obligation that Zhilinskij has taken on with France, namely the launch of a major offensive against Germany simultaneously with the first blow fired against Austria. The Russian Empire has only recently resolved some important questions regarding the conduct of the war that has been entrusted to the Grand Duke Nicholas. In fact, at the outbreak of hostilities, Tsar Nicholas II intended to personally take over the conduct of the operations despite being completely lacking the necessary capabilities. The pressure of his ministers convinces him, albeit reluctantly, to entrust the task to his uncle. The Grand Duke Nikolaj Nikolaevič Romanov is highly regarded within the armed forces. Since the beginning of August 1914, Grand Duke Nicholas has been pressed by the French who ask him to begin hostilities to reduce the pressure on their front. It is from these premises that the invasion of East Prussia was born, carried out without the Tsar's army having had the opportunity to prepare it adequately. The conduct of the operations, in reality rather clumsy, is entrusted, as we have seen, to General Žilinskij who commands the army group under Generals Rennenkampf, at the head of the 1st, the so-called Vilna Army, and Samsonov who leads the 2nd also known as the Warsaw Army. The plan is simple. Rennenkampf must move first, attacking the eastern edge of Prussia in order to attract the bulk of the German forces to that side. 48 hours later Samsonov will also go into action who, having crossed the southern border, will have to strike the rear of the German forces, cutting off their contact with the Vistula line and thus squeezing them in a vice. The plan is potentially very effective, so much so that it causes great concern among the Germans, but it presents intrinsic elements of weakness. We have already seen the first: the state of unpreparedness of the troops and the serious limitations of Russian logistics. But there is another represented by the orographic factor, which in military history always has a significant weight: the two armies of Rennenkampf and Sansonov are separated by the Masurian Lakes, a strip of about eighty kilometers that prevents them from having any contact. On August 17, Rennenkampf crosses the border with six and a half infantry divisions and five cavalry divisions. The Germans had hypothesized that an action of this kind could take place and the countermeasure identified was precisely that of exploiting the natural obstacle of the Masurian Lakes to concentrate all forces against the first wing of the Russian deployment that appeared on the battlefield and - having annihilated that - then deal with the second. Commanding the German operations in East Prussia was General Maximilian von Prittwitz who, on August 20 - near Gumbinnen - launched a frontal attack with the bulk of his forces against Rennenkampf's forces, which were advancing slowly. At first von Prittwitz had thought of waiting for them along the line of the Angerapp river but then, also pushed by pressure from General Hermann von François, commander of the 1st Army Corps, he went on the attack. He achieved successes on the wings of the front but in the center, August von Mackensen's XVII Corps suffered heavy losses - also due to Russian artillery fire - and was forced to retreat. Von Mackensen was supposed to deal the Russians the hardest blow and the beating he suffered, especially on a psychological level, also cancelled out the positive results obtained on the wings of the attack front by von François. The Germans retreated but Rennenkampf did not pursue them. His troops are tired and hungry for lack of supplies. Meanwhile, beyond the Masurian Lakes, Samsonov has reached the border with Prussia. He did so under pressure from Zhilinsky but his forces are also tired and without supplies...