Many restrictive rules also have been accepted tending in a small degree to mitigate the actual operations of war, and they have had some real influence in this direction, though it is not possible to justify the military code on any clear principle either of ethics or logic. Assassination and the encouragement of assassination; the use of poison or poisoned weapons; the violation of parole; the deceptive use of a flag of truce or of the red cross; the slaughter of the wounded; the infringement of terms of surrender or of other distinct agreements, are absolutely forbidden, and in 1868 the Representatives of the European Powers assembled at St. Petersburg agreed to abolish the use in war of explosive bullets below the weight of 14 ounces, and to forbid the propagation in an enemy's country of contagious disease as an instrument of war. It laid down the general principle that the object of war is confined to disabling the enemy, and that weapons calculated to inflict unnecessary suffering, beyond what is required for attaining that object, should be prohibited. At the same time explosive shells, concealed mines, torpedoes and ambuscades lie fully within the permitted agencies of war. Starvation may be employed, and the cutting off of the supply of water, or the destruction of that supply by mixing with it something not absolutely poisonous which renders it undrinkable. It is allowable to deceive an enemy by fabricated despatches purporting to come from his own side; by tampering with telegraph messages; by spreading false intelligence in newspapers; by sending pretended spies and deserters to give him untrue reports of the numbers or movements of the troops; by employing false signals to lure him into an ambuscade. On the use of the flag and uniform of an enemy for purposes of deception there has been some controversy, but it is supported by high military authority.[29] The use of spies is fully authorised, but the spy, if discovered, is excluded from the rights of war and liable to an ignominious death.