In An essay towards the present and future peace of Europe, by the establishment of a European diet, parliament or estates, published in 1693, William Penn envisaged constitutional arrangements for a United States of Europe.

Now if the sovereign princes of Europe, who represent that society or independent state of men that was previous to the obligations of society, would for the same reason that engaged men first into society, viz, love of peace and order, agree to meet by their stated deputies in a general diet, estates, or parliament, and there establish rules of justice for sovereign princes to observe one to another; and thus to meet yearly, or once in two or three years at farthest, or as they shall see cause, and to be styled, the Sovereign or Imperial Diet, Parliament or State of Europe; before which sovereign assembly should be brought all differences depending between one sovereign and another that cannot be made up by private embassies before the sessions begin: and that if any of the sovereignties that constitute these imperial states shall refuse to submit their claim or pretensions to them, or to abide and perform the judgment thereof, and seek their remedy by arms, or delay their compliance beyond the time prefixed in their resolutions, all the other sovereignties, united as one strength, shall compel the submission and performance of the sentence, with damages to the suffering party.