When Military Force Is Legal
Under international law, a country may generally use military force only with UN Security Council authorization, in self-defense against an armed attack, or with the valid consent of the country where the force is used.
It's often said that a country can legally attack whenever it believes another government is dangerous — in fact, fear or political disagreement alone does not normally create a legal right to use force.
The main framework is the United Nations system, where the Security Council is meant to handle threats to international peace and can authorize collective action. Separate rules also govern how forces must behave once fighting begins, including protections for civilians and prisoners.
Think of countries as neighbors who agree not to break into each other’s homes or start fights. A neighbor may act to stop an attack, invite help into their home, or follow a decision made by the building’s agreed-upon committee.
Knowing these rules helps when headlines describe an invasion, airstrikes, peacekeeping mission, or claim of self-defense, and when people argue whether a military action is lawful or merely politically justified.
Say Country A is attacked by Country B, and Country A uses limited force to stop the attack while notifying the United Nations. That may fit self-defense, but if Country A continues a wider campaign after the threat has ended, the legal justification becomes much harder to support.
Self-defense has limits
Even when self-defense is available, the response is generally expected to be necessary and proportionate to stopping the attack rather than serving as punishment or unlimited retaliation.
Consent can change the analysis
Foreign troops may have a different legal basis when the government controlling the territory clearly invites them, although that invitation can be disputed if the government lacks authority or is acting under coercion.
Lawfulness has two stages
A separate body of rules governs how a conflict is fought, so having a lawful reason to begin using force does not permit attacks on civilians or other prohibited conduct.