The King initiates the play's action when he insists that his friends swear not to see women. Once the men fall in love, the King looks to Berowne for justification in breaking their oath, but he's the one to give the starting signal of romantic pursuit.
Some critics argue that Berowne is the protagonist. He's a more interesting, dominant character and has more lines. There's a sense in the opening scene that if he hadn't agreed to the vow, all bets would be off. The exposure of Berowne's letter and identity as a lover sets off the chain of events that allows the men to woo the women.