He had none of his father's fanaticism but was intelligent and tolerant; in better times he would have been a good Emperor. He proved courageous and cool-headed in battle, and in 1634, played a decisive role in the defeat of the Swedes at Noerdlingen. Between 1637 and 1643 he tried in vain to end the war with the Protestants but was prevented by France and Sweden, both wanting to humiliate the Empire.
Negotiations took five long years while fighting continued before the peace treaty was signed in 1648 at Muenster. After 1648 there were only nine years left to him, which time he spent trying to help the Empire recover from the destructions of war.
 

Source: Leo van de Pas