What's the main difference between "announce" and "declare?" Are they interchangeable?

Native speaker’s answer
Rebecca
"Announce" means to give notice of something publicly, usually for the first time. "Declare" means to make something known formally or officially. They are interchangeable at times - I would say that a "declaration" is a form of an "announcement," just more formal. But an "announcement" is not always a "declaration." Ex: Our team was declared the winner of the tournament. Ex: They announced that the games are postponed by a month.