The curtain rose on the opening act of a new era.
That incident became the prologue to the war.
The ceremony's curtain-raiser began with the national anthem.
His election opened the curtain on the reforms.
This incident marked the opening act of the war.
プロローグ is the borrowed English word for a prologue, used interchangeably with 序幕 in more casual or modern contexts.
発端 is a neutral term for the origin or trigger of an event, while 序幕 carries a theatrical, dramatic connotation of an opening act.
終幕 is the closing act or final curtain, the direct opposite of 序幕, which is the opening act.
From 序 (jo, 'beginning') + 幕 (maku, 'curtain'), referring to the opening curtain of a theatrical performance.