Battle of Tannenburg
1410 July 15, The Grand Duchy of Lithuania and the Kingdom of Poland join forces to fight the Knights of the Teutonic Order. The battle of Tannenberg would begin on that day, one of the most decisive battles in the Polish-Lithuanian-Teutonic Wars and one of the greatest battles in the Middle Ages.
Armies from both sides gathered near the plains between the villages of Grunwald, Staebark and Łodwigowo.
To explain why these armies gathered, we must look back, to the 13th century, when the Teutonic knights were sent against the pagan Prussians. The Teutonic order soon had set its sights on another pagan country - Lithuania, once Prussia, (now under the rule of the Teutonic order) gained a "Christianized" state."
Holy crusades and invasions of Lithuania were formed, most of them being beaten back.
In 1385, The Lithuanians and Polish were joined by the union of Krėva. Both knew they needed to work together to destroy the power of the Teutonic order. Jogaila, Grand Duke at the time, married Monarch Jadwiga of Poland. Jogaila accepted Christianity (and became the King of Poland - Władysław Jagiełło) and christened Lithuania. The Teutonic Order Had lost its justification to exist.
However, the Teutons invaded again in 1398 and both nations suffered, unprepared to fight the Teutonic Order.
In 1409 an uprising started in a province of the Teutonic Order. Jogaila said he would stand by his promises to defend both nations against their enemies. Ulrich von Jungingen, the Grand Master of the Teuton Order used this as a pretext to declare war on Poland and Lithuania. The forces of the Order invaded Poland, but were routed by the Poles, who soon retook Bromberg (Bydgoszcz).
Foolishly, Ulrich von Jungingen gave more time for Lithuanians to gather their forces, while he awaited European mercenaries. On June 30 The forces of Poland crossed the Vistula over a pontoon bridge and joined with the forces of Masovia and Lithuania. The Lithuanian forces were commanded by the King's cousin - Vytautas the Great, who would play a great role in the battles to come. Some historians, however, lower his role in the anti-Teuton campaign.
On July 9, 1410 both forces invaded the Teutonic Orders lands and the Knights were caught by surprise, because they expected attacks separately and aiming towards different targets.
Soon, two castles were captured, a few towns plundered and the way into Teuton-Prussia was open, as the Polish-Lithuanian forces went towards the enemies headquarters - Marienburg.
Now, forward to July 15, 1410.
The Right Flank of the allied army was guarded by Vytautas of Lithuania, his cavalry consisting of Lithuania's Riders, Tartars and Moldavians. The army of Polish-Lithuanians outnumbered the Teutons, but weren't as well equipped. The Overall commander of the Polish was Jogaila, Vytautas commanding the Lithuanian units.
At dawn, both forces stood against each other. The battle started at Noon with an all out assault by Lithuanian cavalry supported by some polish cavalry units. Soon they faked a retreat towards the marshes and woods. This tactic was often used by the Mongols in east Lithuania.
The Teuton cavalry, confidently chased after the retreating Lithuanians, but were stopped by the marshes, while Lithuanians regrouped. At the moment, heavy fighting was happening on the left flank, where the strongest polish unit was fighting Ulrich von Jungingen, who had led a cavalry charge against the left flank. The Poles sustained heavy casualties and had to call in their reserves. The Teutons were beaten back and weakened. At the same time, the Teutons were still trying to chase the Lithuanian cavalry; they were too late to help the other Teutons and withdrew to their own lines.
After several hours, Ulrich rejoined the main line of engagement. Vytautas by this time has returned to the bloody battlefield and joined the fighting between the Teutons and Polish. By this time the Teutons were extremely outnumbered by Lithuanian infantry, who were coming from the forests, and Polish knights. Ulrich himself led 16 units of cavalry, who were kept in reserve. Jogaila ordered in all of his troops, fresh and tired ones, into the fight. The Teuton units were surrounded and easily slaughtered by Poles and Lithuanians. Ulrich von Jungingen was also cut down in action. The rest of the Teutonic forces routed and returned to their camp. Some troops went into the forests, but were killed by Lithuanian cavalry (It should be noted that Lithuanians were very experienced at fighting in forests.)
Artillery was also deployed in this battle, but it failed to turn the tide of battle.
After this, the Lithuanians and Poles marched on to Marienburg, but winter came and they were forced to retreat.
Many recordings of this battle are found, but none should be trusted completely as they contradict themselves, most having over-exaggerated amount of soldiers, units, etc.