Horses played an important role in warfare during three millennia. In this first part of two on ‘horses in wartime’, a succinct description is given of the very diverse use of different types of horses from the late mediaeval period on to the eve of the First World War. At first, these animals were crucial on the actual battle fields in the famous cavalry charges, where they were able to crush infantry troops. This changed definitely after the introduction of fire weapons, but horses continued to be needed in high numbers for transport and logistics. Causes of the often heavy losses in animals, not less than in humans, are briefly discussed. Malnutriton and inefficient care of the healthy as well as of the sick were more important causes of mortality than deadly wounds or shooting lesions incurred during battles. This is illustrated most convincingly by the terrible debacle of the invasion of Russia by Napoleon.