The 1969 Vienna Convention leaves to States parties a treaty invalid because of its conflict with a peremptory norm the initiative, and the choice of having the International Court of Justice declare the invalidity or of reaching an agreement to the same result. The Vienna Convention provides a similar solution with regard to the invalidity of treaties concluded as a result of coercion. According to widespread opinion, third States may not consider those treaties invalid independently from the parties' action. This outcome is particularly problematic, given that both are cases of so-called absolute invalidity, where nullity is the consequence of a contrast with rules of fundamental importance in contemporary international law. This chapter explores ways in which third States may invoke such an absolute invalidity, reaching the conclusion that through the well-established practice of nonrecognition States have long declared their intention to consider treaties in such cases invalid, notwithstanding any lack of initiative of the States parties.