An act of war is an aggressive act committed by one state against another. It is most commonly used to refer to hostile actions by a government during peacetime that give other states a justification for declaring war, or casus belli. During wartime, it refers to actions carried out by governments to wage that war. In some contexts, it can also include military actions carried out by non-state actors, such as rebels, terrorists, or partisans. The concept of an act of war is important to international politics and diplomacy and can also be relevant in other areas, such as the insurance industry.
A number of different actions can be an act of war besides the obvious example of an an actual invasion or other military strike. A blockade, the use of military force to cut a nation or part of a nation off from outside trade or supplies, is an act of war against the blockaded country. Violent covert operations, such as assassinations or sabotage carried out by a government's agents in foreign countries, can also be considered acts of war if discovered. The idea of actions during peacetime that constitute acts of war justifying retaliation is often important to ethical theories about when war is morally acceptable, such as the idea of just war in Catholic philosophy, and in international treaties that restrict the use of warfare as a means of resolving disputes.
Acts of war during wartime are regulated by a body of law commonly called the laws of war. For example, under the Geneva Convention, combat forces lawfully engaging in acts of war are required to bear arms openly and identify themselves as combatants. Blockading an enemy port to prevent the importation of war supplies is considered a legitimate military tactic, but some particular practices are not. For instance, under modern international law a ship from a neutral country entering a blockaded port can be boarded and inspected for contraband, by force if necessary, but the blockading force cannot simply open fire on a neutral ship for approaching the blockaded port. The specific rules imposed by the laws of war have varied over time, created by a combination of accumulated customs, legal precedents, and treaties, and the extent to which they are actually followed also varies.
Losses caused by acts of war are often not covered by insurance, such as homeowner's or life insurance, because many insurance policies have clauses specifically excluding them. This usually includes losses suffered due to terrorist attacks, insurrections, and civil unrest as well as those caused by belligerent states. It is possible to buy insurance that does cover acts of war, called war risk insurance. These policies are bought primarily by international companies operating in countries where there is a serious risk of property damage or employee injury due to political instability or violence.