The Peace of Westphalia in 1648 was extremely important not only because it ended the 30 years war but because it changed the ideology of warfare after that point. Until the Peace of Westphalia most of the conflicts in the west were religion based. After the Peace of Westphalia that entirely changed.
This website gives a very good description of the background of the Thirty Years War and the Peace of Westphalia. It also discusses some of the outcomes and effects on Europe as a whole: