Battle of White Mountain in November 1620 was the first big battle of the 30 Years War. The painting depicts a celebrated Catholic victory in the Thirty Years War over heretical Protestant rebels with the Protestant rebels defending Prague against the Catholic Holy Roman Emperor. Though the war lasted another 27 years, this battle, in the words of Wikipedia, determined who would control Czech lands until the end of WWI.
If you take a closer look you can see it more on an individual level, like when we see a cannonball smashing a soldier to bits, we understand it as part of the great Catholic victory. If the soldier is fighting on the Protestant side, however, his death is a just reward for rebellion and heresy. If the soldier is fighting in the Catholic army, his death is a noble sacrifice for a worthy cause. If we look up, we can see angels hovering high above the battlefield. They are holding a billboard which explains in Latin what happened in this battle, and why it was so important. The message is that God helped Emperor Ferdinand II defeat his enemies on 8 November 1620.