To Proclaim
する verb
If 宣言 is a "proclamation," then the verb version will be to proclaim.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
あの時、日本政府は非常事態を宣言するべきだったと思わないかい。
Don’t you think that the Japanese government should have declared a state of emergency at the time?