Some proponents of the legality of the invasion argued that UNSC authorization was not required, since the invasion was an act of collective self-defense provided for under Article 51 of the UN Charter. Specifically, it was argued that a series of UN Security Council Resolutions concerning Afghanistan provided for the possibility of establishing that the Taliban were indirectly responsibility for al-Qaeda's attacks, on the basis that Afghanistan was offering them safe harbour. Some critics claimed that the invasion was illegal under Article 51 because the 9/11 attacks were not "armed attacks" by another state, as required under article 51 of the Charter: they were not perpetrated by Afghanistan but by non-state actors. They argued that the actions taken by the terrorists in 9/11 were not attributable to Afghanistan. This position is consistent with the case law of the International Court of Justice, also known as the World Court, which has been slow to recognize attacks carried out by non-State actors as attributable to States, even in cases where States lend their support to the actions of non-State actors. Others claimed that, even if the 9/11 attacks were attributable to Afghanistan, the response of the NATO coalition would not constitute self-defense as these acts do not meet the proportionality test under international law, as established in the Caroline Affair.