Union Pacific Railroad Co. v. Public Service Commission of Missouri, 248 U.S. 67 (1918)

Compliance under threat of penalty is duress, not voluntary conduct: 'as conduct under duress involves a choice, it always would be possible for a State to impose an unconstitutional burden by the threat of penalties worse than it in case of a failure to accept it, and then to declare the acceptance voluntary' (at 70); 'the application for a certificate and the acceptance of it were made under duress' (at 70). Decided Dec. 9, 1918 (argued Nov. 19-20, 1918).