포문을 연 is a attributive adjective form of 포문을 열다 (열다 = to open, 연 = open; which has already opened). 포문을 열다 literally means to open the barrel of an artillery guns(포문), i.e. to start firing. It is however more commonly used as an idiom for verbal or other assaults. So 포문을 연 describes a person or group who has started an aggressive act. For example, - 한국팀은 김 선수의 골로 포문을 열었다 = The Korean team scored the first shot with Kim's goal. - 김 선수의 골로 포문을 연 한국팀은 두 골을 더 넣으며 승리했다 = The Korean team, which opened its salvo with Kim's goal, went on to win the game scoring two more goals.