What Does It Mean When You Declare No Fly Zone Over Ukraine?

When a nation imposes or declares no flys on it is not enough to declare it. You have to enforce it. Ukraine is right now controlled more or less by russia, right? And we know that they have achieved air superiority over ukraine in the first few hours of the war. So that's what we know. Now if the americans were to declare no fly zone over ukaine, it would mean that they would say, nobody is allowed,. no plane is allowed to enter the ukrane and air space without american permission. But ukrane, the air space, is already controlled by russia. It means you are actually going to war