This is the 6057th most frequent Korean word.
선언하면서
"While declaring" or "as declaring."
The word '선언하면서' here means 'while declaring' and depicts an action occurring simultaneously with the main verb '시작했다'—to begin.
그는 자유를 선언하면서 연설을 시작했다.
He began the speech while declaring freedom.
In this sentence, '선언하면서' indicates the concurrent action of declaring independence as he begins a new venture.
직장에서 독립을 선언하면서 그는 자신의 사업을 시작했다.
While declaring independence from his job, he started his own business.
Here, '선언하면서' is used to show the simultaneous action of asserting self-confidence as the child participated in a sport.