"henceforth declare" is a correct and usable phrase in written English.
You can use it when you want to emphasize that a certain statement is a declaration of an authoritative decision. For example, "Henceforth declare, no person shall be denied access to education."
"Henceforth," declared Senator Mitch McConnell of Kentucky, the Republican leader, "I will always think of him as 41".
"I have, with much reluctance, assumed executive authority of the country and henceforth declared martial law," he said at a news conference.
However, the Commission Sportive Internationale intervened to declare that henceforth fan cars would not be allowed and the car never raced again in Formula One.
Henceforth, the government declared that Canada's mass media would be required to encourage Canadian content books, television programs, magazines, and other locally made cultural products.
In France, where the Catholic church was the sole faith, the revolutionnaires detested God as a crucial part of the ancien régime: politics, they declared, henceforth would be protected from this evil.
Henceforth, the ministry declared in a 4 March statement, Vulpes vulpes kurdistanica, a Turkish subspecies of the red fox, will be referred to simply as Vulpes vulpes.
Nevertheless, a deterministic model will henceforth be assumed whenever we declare the transmission power for the Nakagami model, that is, the power necessary to reach a communication range of meters.
But also he would be a double-or-quits choice for the Tories: if, in a referendum, Britons were to vote to join he would either have to quit as leader, recant, or declare that he would campaign henceforth for withdrawal.
All he has to do is declare that his administration will henceforth act to block taxpayer funds from ever again going to this very openly radical and socialist organization.
Barring such a sensible, sweeping approach, the FCC should, among other things, declare all broadband services, such as cable, to be henceforth "information services" and thus not subject to price controls or political manipulation.
