Mutually agreed-upon rules that define how military forces should behave during times of war, including the treatment of prisoners and civilians. Rules of engagement differ from a warrior's code, which is a less formal set of defined limits on what warriors can and cannot do if they want to continue to be regarded as warriors, rather than murderers or cowards. For the warrior who adheres to such an informal code or to the formal rules of engagement, certain actions are unthinkable, even in the most dire or extreme circumstances. Rules of engagement date at least to the Middle Ages in Europe, when highly trained and well-paid gentleman knights spent years in apprentice and training to prepare them to wage war.They wielded heavy weapons and they, and their steeds, wore heavy armor for protection against those same weapons wielded by their noble opponents in battles fought on the field of honor.The invention of the crossbow upset the balance, however, as one small bolt from a crossbow fired by even the least skilled, most common peasant farmer could topple even the mightiest and most gentlemanly knight wearing the heaviest armor. Once toppled to the ground, the knight became immobile and, therefore, an easy kill for a common peasant with a stiletto. This innovation was considered so disgraceful that Pope Innocent II in 1139 declared the crossbow "hateful to God and unfit for Christians." The second Lateran Council of churches stated that, "We prohibit under anathema that murderous art of crossbowmen and archers, which is hateful to God, to be employed against Christians and Catholics from now on." This decree did not prohibit the use of the crossbow against infidels, who apparently weren't considered to be gentlemen, much less worthy of the protection of the Church.The Church of England also attempted to outlaw the crossbow. See also Geneva Convention and warrior's code.