What is an example of an illegal target in warfare?

An illegal target in warfare refers to a target that cannot be lawfully attacked under international humanitarian law and the laws of armed conflict. Historic buildings are considered illegal targets because they hold cultural, historical, or religious significance. Under the Hague Convention and customary international law, attacks on such sites are prohibited due to their cultural value and the principle of distinction, which obligates parties in conflict to differentiate between military objectives and civilian objects.

In contrast, enemy troop formations represent legitimate military targets as they contribute directly to the enemy's military capability. Similarly, supply routes are crucial for the operational effectiveness of military forces and are therefore acceptable targets in warfare. Friendly military units are also not lawful targets, as attacking one’s own forces is considered a violation of military conduct and would undermine operational effectiveness.

Thus, the identification of historic buildings as illegal targets is grounded in the protection of cultural heritage and the rules governing armed conflict, making it the correct answer.