The three legal principles that govern how wars must be fought

What is the goal of international humanitarian law?

The laws of armed conflict, often called international humanitarian law, do not try to outlaw war. Instead, they accept that wars happen and attempt to set boundaries on the violence. The framework is built on a simple, pragmatically brutal compromise: militaries are allowed to wage war, but they must minimise the suffering of those who are not fighting.

Almost every country has signed up to the core treaties, chiefly the four Geneva Conventions of 1949 and their subsequent Additional Protocols. These rules apply to all sides in a conflict, regardless of who started it or why.

Distinction: Knowing who is a target

The first and most fundamental rule is distinction. Combatants must always distinguish between those who are fighting and those who are not.

Under this rule, weapons can only be aimed at military targets—such as enemy soldiers, weapons systems, and military bases. Civilians, hospitals, schools, and residential buildings are strictly off-limits.

However, civilian structures can lose their protected status if they are being used for military purposes. If a school is turned into a weapons depot, it becomes a valid military target. But even when a target is legitimate, commanders cannot simply attack it without considering the next two principles.

Proportionality: The calculation of civilian harm

Proportionality is perhaps the most misunderstood concept in the laws of war. In everyday language, "proportional" suggests an eye-for-an-eye response. In international law, it means something entirely different.

A military strike is illegal if the expected harm to civilians or civilian property is excessive compared to the concrete and direct military advantage anticipated from the attack.

This is a calculation made before a strike occurs, based on the information available to commanders at the time. It does not mean civilian casualties make an attack automatically illegal. If a strike destroys a major enemy command post, a high level of collateral damage might be deemed legally "proportionate" under this definition. If the target is a single sniper, the same level of civilian harm would almost certainly be a war crime.

Precaution: The duty to minimise risk

Even when a target is legitimate and the attack is deemed proportionate, commanders still have a duty of precaution. They must take all feasible measures to avoid or minimise civilian harm.

In practice, this means military forces must verify that their targets are actually military objectives before attacking. They must choose weapons and tactics that limit civilian risk—for instance, using precision-guided munitions instead of unguided bombs in populated areas when possible.

Precaution also requires giving effective advance warning to civilians before an attack, unless the element of surprise is essential to the mission's success.

How are these laws enforced?

These rules are not just moral guidelines; they are binding international law. When they are broken, individuals can be prosecuted for war crimes.

Enforcement usually falls to national courts first, as states have the primary responsibility to discipline their own troops. When national systems fail or refuse to act, international bodies like the International Criminal Court (ICC) in The Hague can step in to prosecute commanders and political leaders.

Proving a violation in court is exceptionally difficult. Prosecutors must prove not just that civilians died, but that commanders knew the civilian harm would be excessive, or failed to take precautions, or targeted civilians intentionally. Because military decisions are made in the "fog of war," the legal standard relies heavily on what a reasonable commander would have known at the moment the decision was made.