Outnumbered and with many inexperienced troops, Tilly built defensive works along the River Lech, centred on the town of Rain, hoping to delay Gustavus long enough for Imperial reinforcements under Albrecht von Wallenstein to reach him.

Instead, he moved into Bavaria from his winter quarters in Mainz, where by combining his own forces with those led by Horn, Johan Banér, and William, Duke of Saxe-Weimar, he assembled around 37,500 men and 72 guns.

Although this meant Gustavus could outflank these positions by passing south of Augsburg, Tilly hoped this would provide time for the main Imperial army under Albrecht von Wallenstein to reach him.

[1] At this point, the Lech river divided into a number of parallel, fast flowing streams, each about 60 to 80 metres wide; the bridge at Rain had been destroyed by Tilly, who placed his inexperienced troops in a strong redoubt with 20 guns, making it a formidable challenge for an attacker.

Tilly's right thigh was shattered early in the battle; he was taken unconscious to the rear and died two weeks later, [7] while his second in command, Johann von Aldringen, was temporarily blinded minutes later.

The country was extensively pillaged, while Gustavus made a triumphal entry into Munich on 17 May, confiscating the ducal art collection and capturing over 100 pieces of artillery; it was another three years before Maximilian re-entered his capital.

[10] On the other hand, the Swedes were now at the end of long and extremely vulnerable supply lines, while Bavarian peasants waged a bitter guerrilla war in the countryside against the invaders plundering their lands.

[8] Meanwhile Wallenstein raised an Imperial army of 65,000, which he used to expel the Saxons from Bohemia; concerned Saxony might make a separate peace and leave him isolated, Gustavus now summoned his German allies to Nuremberg.