"English and American courts have come to regard it as 'an axiom of international law' that foreign states should be immune from suit in the national tribunals unless they to the expressly or impliedly waive their immunity and submit to the jurisdiction.... Yet it has not been doubted that states may waive immunity and submit to the local jurisdiction if they wish. In practice they frequently find it advantageous to do so. Some difficult questions arise when it becomes necessary to define the requisites of a waiver or to determine its precise effect in a particular case."
Dickinson, Edwin D. "Waiver of State Immunity." Am. J. Int'l L. 19 (1925): 555–9.