Preemptive war
A preemptive war is one in which a country takes military action against another country or group that poses a real threat of waging a war against them. The purpose is to stop the threat before the country or group has a chance to strike. This is an old philosophy of war or of preventing a war. Sun Tzu, the ancient Chinese general, military expert and philosopher, said a supreme country would break resistance in its enemies without needing to fight them, but if fighting is needed, then a country should consider attacking first if doing so would gain them an advantage (since that increases their chance of ending the conflict quickly). A declaration of war may or may not come before the preemptive first strike.  Preemptive war is thought of as a defensive act. This is true when it is clear an enemy is threatening an attack and that attack will do great harm. Also, a country which preemptively attacks another to defend itself can claim it was a moral act and avoid the label of an aggressor.