In war, there are in principle legitimate military targets and illegitimate civilian targets. If civilians or their property is affected, it is nevertheless not a war crime if a) that effect could not be foreseen or b) it could not be avoided and was minor in relation to the military objective. Clearly, it is often difficult to judge in individual cases whether civilian loss was a war crime or collateral damage.
By the laws of war (see also international humanitarian law), civilians and medical personnel are not to be made the deliberate targets of military operations (see attacks on humanitarian workers). Collateral damage includes accidental damage to non-military property, particularly civilian homes. However, deliberate damage to transport and communication infrastructure is a legitimate military objective and neither war crime nor collateral damage. Determining what classes of target are of military value and thus acceptable to attack is a hotly debated topic.
The line between collateral damage and atrocity is sometimes blurred. For an offensive strategy this may happen when deliberate attacks are made despite knowledge of some civilian presence, disregarding civilian lives in favor of a military objective. An opposite defensive strategy is to actively use civilians as human shields by locating military facilities nearby or within civilian neighborhoods or hospitals. This latter case is a clear case of abuse of the laws of war intended to protect civilians. It is the subject of dispute whether an attacker is entitled to proceed with military activity despite the civilian presence in such cases.
Collateral damage may also be ecological or environmental, affecting a human population less directly but often more severely. During the Gulf War there was very substantial damage to the Persian Gulf and Kuwait desert ecology due to deliberate releases of oil and the deliberate demolition of oil wells (part of a scorched earth strategy) - the Kuwait oil fires took many months to put out. In addition, the movement of huge numbers of troops and equipment through the thin soil of the region made farming difficult to resume after the war. This in turn displaces refugee populations which suffer serious health hazards due to migration, or living in a refugee camp. Destruction of olive trees in the Gaza Strip by Israel is also classified by some as collateral damage of the intifada.
See also: Civilian, war, laws of war, civilian casualties, non-combatant, environmental roots of conflict. Bombing of Dresden in World War II. Hiroshima & Nagasaki, Bombing of Tokyo in World War II, Nuclear warfare