to warn - to make someone conscious of a possible problem or danger so that they will not be hurt.

to declare - to announce officially that something is true or happening.

Australia declared its support for the agreement.
I was in Germany when war was declared.
A state of emergency has been declared in the province.

to announce - to make a public or official statement, especially about a plan, decision, or something that has happened.

There was a press release announcing the Senator's resignation.
Exxon has announced a 26% increase in profits.
I am pleased to announce that the Board has agreed to create 500 new jobs in our sales division.

to express - to tell someone about a feeling, opinion, or aim by speaking or writing about it.
Also used about a letter, book, or other piece of writing.

To declare offers more of an individualized expression or opinion than to announce it as if it's a done deal.