They just did it with the ministry of magic, people like Nott and Rookwood used mind control, blackmail, and influence peddling to take the reins over wizards in Britain, and did so in a relatively short time of about a year and a half from Voldy's public reveal at the Department of Mysteries. They know it's effective its just a matter of how much research they have to do to find the right Mr. Simpkins at water and power, so give it 5 years and the Death Eaters and their Ministry of Magic collaborators (Delores Umbrige Et Al.) will have something resembling total government control.