The Battle of Lützen was one of the most decisive battles of the Thirty Years' War. It was a Protestant victory, and nearly cost the life of one of the most important leaders of the Protestant alliance, Gustavus Adolphus of Sweden. Had he perished during the course of the battle, many historians suggest that the Protestant cause would've lost direction.
Wallenstein had split his forces and largely withdrawn from Germany as the winter months wore onward, expecting the war to draw to an end for the year. Gustavus Adolphus expected this and sought to capture him as he retreated. As his forces moved to assault the withdrawing forces of Wallenstein, they stumbled upon a rearguard group, and were engaged in a skirmish which kept the Swedish forces from achieving their goal.
Wallenstein dug in and sent for General Pappenheim to come to his aid. While the Swedes prepared for the morning, so, two did Wallenstein's forces.
A misty morning delayed the initial attack until 9:00, and even then did not see the Swedes in full array until nearly 11:00. In the course of the battle Pappenheim was fatally wounded, greatly demoralizing his troops. A mere two hours after the day's battle began, Gustav Adolph was himself grazed. His troops found him, and initially thought him dead, having been in part stripped by Wallenstein's forces.
The news of his fall was told to the Swedish troops who rallied, carrying the day in a decisive victory. It was only later that night that the Swedish troops were able to rally around their King who they thought dead.
With Gustav Adolph surviving and the Swedes carrying the day, Wallenstein was successfully swayed to the Swedish side. His defection from the Habsburg forces lead to the Battle of Tabor and the Battle of Pisek and the eventual Sack of Vienna