The Thirty Years War began as a religious war between Protestants and Catholics within the Holy Roman Empire but quickly escalated as the European great powers intervened to protect their own interests and to preserve the balance of power. Estimates of the numbers of deaths vary but most indicate that the populations of the German states and the Czech lands declined by at least a third.
Many civilians were killed by marauding bands of troops, others died from famine or disease, or they were forced to abandon their homes. One of the most notorious atrocities happened in 1631 after the siege of Magdeburg in Saxony – a mainly Protestant city attacked by Catholic forces. When the attackers entered the city, not having been paid for months, they looted everything they could find and massacred people when they had no more possessions. The atrocities committed there led to the term Magdeburgisieren (or Magdeburgisation) being used for many decades afterwards to describe any similar military atrocity. The atrocities and the devastation of the Thirty Years War created widespread public revulsion and demands for clearer and consistent rules of combat and rules on the protection of civilians in times of war.