In addition to regional movements, "supranational organizations that promote progressive integration between its members started appearing in the second half of the 20th century. Most of these organization were inspired by the "European Union["clarification needed], and while member states are often reluctant to form more centralized unions, the concept of unionism if often present in public debate.
The political position of the "United Kingdom is often discussed; and former states like "Serbia and Montenegro (2003–2006), the "Soviet Union (1922–1991) and the "United Arab Republic (1958–1961).
"Lord Durham was widely regarded as one of the most important thinkers in the history of the "British Empire's constitutional evolution. He articulated clearly the difference between a full legislative union and a "federation. In his "1839 Report, in discussing the proposed union of "Upper and Lower Canada, he says:
Two kinds of union have been proposed – federal and legislative. By the first, the separate legislature of each province would be preserved in its present form and retain almost all its present attributes of internal legislation, the federal legislature exercising no power save in those matters which may have been expressly ceded to it by the constituent provinces. A legislative union would imply a complete incorporation of the provinces included in it under one legislature, exercising universal and sole legislative authority over all of them in exactly the same manner as the Parliament legislates alone for the whole of the British Isles.