Because this rule is part of the law of armed conflict,it assumes that there is an armed conflict. It assumes there are parties to that conflict,that each is fighting for a particular military objective,and that attacks are generally launched in order to gain military advantage. Within that overall framework,some things are allowed (like killing combatants or bombing military facilities) and others are not,like killing too many civilians.
In the case of the almost-attack on Iran,there was (and is) no armed conflict. The laws of armed conflict are therefore not the dominant framework that should have guided Trump’s deliberations over whether or not to go ahead with the strikes (although they would have instantly become a very relevant framework if the strikes were not called off and started a war).
The issue of when it is and isn’t ok to use armed force against another country is governed by a different set of rules,namely,the Charter of the United Nations. The rules are pretty simple. It is illegal for one country to use force against another,unless it is either (a) authorised by the UN Security Council,or (b) in self-defence. There is no other circumstance in which it is ok for one country to launch a military strike against another. International law does allow"countermeasures",but only such measures that fall short of the use of force. There is no right in international law to launch retaliatory strikes,no matter how calibrated,proportionate or targeted.
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There is some debate over whether Iran breached international law when it shot down the drone. If it was in international airspace,it was illegal;if it was in Iranian airspace,then Iran was arguably entitled to shoot it down. But it doesn’t make any difference to the legality of the US retaliation plan. Even if the US drone had been shot down illegally,that still wouldn’t entitle the US to use force in return unless it could argue it was in self-defence – a case it hasn’t tried to make.
The illegality of the proposed US strikes has been glaringly absent both from the political rhetoric and from media reporting of the escalating crisis. The planned attack was clearly conceived of by the US as a retaliatory strike,and this justification appears to have been largely bought by the media and other commentators with no regard for the fact that there is no such thing as a right of retaliation in international law.
The situation brings to mind the retaliatory attack by the US,UK and France last year on Syrian chemical weapons facilities,following a chemical weapons attack by the Syrian government in the town of Douma. The strikes were illegal,but neither the US nor France felt the need to provide any legal justification (the UK tried,but its justification didn’t go down very well with international lawyers),and no one seemed very bothered by the legal technicalities.