The Battle of Rain (also called the Battle of the River Lech) was fought on April 15, 1632 as part of the Thirty Years' War. The forces involved in this conflict were 40,000 Swedish troops under Gustavus Adolphus and 25,000 Catholic League troops under Count Johan Tzerclaes of Tilly.
Gustavus had a bridge of boats constructed across the Lech river near the city of Rain through the night prior to the battle, and in the morning sent three hundred Finnish Hackapelite troops across the river under fire. The Hackapelites dug earthworks for batteries which then protected the rest of Gustavas' army as they crossed the river. As soon as his army had crossed the river, Gustavus immediately and successfully stormed the hill. Tilly was shot in the leg early in the battle and was removed to the rear; his second in command, Aldringen, was knocked unconscious with a skull fracture minutes later. Maximilian I, Elector of Bavaria ordered an immediate retreat to save the now leaderless army, leaving most of the Catholic League's baggage and artillery on the field. The army itself may only have escaped destruction due to storm and high winds blocking roads in the night that followed
The immediate result of the battle was that the grand duchy of Bavaria lay open for occupation by the Swedish army, enabling Gustavus Adolphus to temporarily threaten the Austrian heartland.
The battle is however more interesting from a tactical point of view rather than from its outcome, as the elaborate plan of Gustavus Adolphus to catch the entire imperial army by complex maneuvering was prevented when Tilly was mortally wounded in the battle, resulting in the early retreat of the imperial army. The Swedish battle plan consisted of two major elements:
1. A strong feint attack by a portion of the Swedish infantry with heavy artillery support against Tilly's strongly fortified center behind the river Lech. The intended effect was to attract the full attention of the imperial army and its reserve. The Swedish force succeeded in establishing and fortifying its position on a small island or peninsula close to the imperial side of the river. From this position it was able to repel a series of fierce imperial counterattacks despite being outnumbered.
2. As the imperial army got tied up in desperate attempts to eliminate the Swedish bridgehead, the Swedish cavalry with no opposition or attention from the enemy was able to cross the river 10 km south of the imperial left wing. From this position they intended to outflank the entire imperial army and thus catch it in a position with the river and the Swedish infantry at its front and the Swedish cavalry in its rear and on its flanks.
When Tilly was mortally wounded, the discipline of the imperial army quickly dissolved and the army withdrew before the arrival of the Swedish cavalry. Thus, Tilly's death possibly saved his army from a complete defeat. Nonetheless, both armies suffered considerable losses (3,000 on the imperial side, 2,000 on the Swedish), mostly due to frontal attacks and counterattacks against fortified positions with strong natural defenses.
The battle of Lech proves more than the Battle of Breitenfeld the innovation of Gustavus Adolphus' tactical imagination. His daring frontal attack in combination with the deployment of a large part of his army for the flanking movement has similarities with the tactics of the Duke of Marlborough at the battlefield of Blenheim (situated in the very neighborhood of Rain) or of Frederick the Great at Leuthen. The disciple of Gustavus Adolphus, Johan Banér, also employed a similar battle plan four years later in the battle of Wittstock.