The use of excessive force in war is an all-too-familiar phenomenon that resists an obvious philosophical solution. A principle that prohibits disproportionate use of force is commonly recognized. Yet I argue that an adequate proportionality principle is more difficult to formulate than may appear. There are too many morally relevant considerations to be weighed — especially harms to combatants versus noncombatants, depending on which side they are on — and we have no clear idea how to weigh them. These difficulties are avoided through the dominant understanding of proportionality codified in international law and military practice, which rules out only attacks that intentionally target civilians or that involve negligence in targeting or conduct. We should find it harder to deny that use of force can be excessive despite conforming to these narrow constraints. Specifically, we can clearly identify as excessive the use of force in pursuit of unjust goals. This will still leave a range of hard cases in which there is a just cause for war. For these cases, I propose a "golden rule" test of the sincerity of deliberation about whether a use of force would be excessive. Relying on narrow constraints that would govern use of force in war can be morally worse than not having them. The golden rule test can help to direct us toward broader, reasonable moral considerations regarding excessive force.