Query: To speed his retreat to Egypt, Napoleon ordered the poisoning of sick men with what substance?
Context: He failed to reduce the fortress of Acre, so he marched his army back to Egypt in May. To speed up the retreat, Bonaparte ordered plague-stricken men to be poisoned with opium; the number who died remains disputed, ranging from a low of 30 to a high of 580. He also brought out 1,000 wounded men.