I’ sure there is already a post about this, but I didn’t found it, so:
Yesterday playing ST-BW we had a divergence about the Strategy Events cards:

1º opinion – As it says in the card “place in your play area” and only in the end “you must discard this card if you execute another strategy card”, I believed that you can have MORE THAN ONE strategy card, BUT, when you execute one (for example, Defensive Strategy or Aggressive Strategy) you then LOSE the other strategy cards, but you only have to make that option when you actually use the card, NOT when you put it in the play area.

2º opinion – placing the card in the play area is executing it, so you IMMEDIATELY lose all other strategy cards.

Is there an official ruling about this?

On strategy card: You must discard this card if you execute another Strategy Event card.

In play one stategy card..when play another discard the first.

Sorry Octobus, I didn't understood:

Are you of the 1º or 2º opinion?

2º opinion – placing the card in the play area is executing it, so you IMMEDIATELY lose all other strategy cards

Here's the quote which explains everything:

8. Play Event Cards.

Players now simultaneously (and
secretly) read the Event cards they received (and placed
facedown in their play area ) during the Execution Phase.
Then, in clockwise order starting with the first player, each
player may – if he wishes – choose and execute one of his
Event cards. After a player has executed his Event card (or
elected not to execute one), his remaining Event cards (if
any) are revealed and discarded without effect.
After an Event card has been executed, it is discarded unless
it instructs the player to place it in his play area . Such
cards are placed next to the player’s Faction Sheet (in his
play area), and may be used when instructed by the card.

BTW: Notice that all Event Cards drawn during Execution Phase are considered to be already placed in Play Area, but placed facedown. It means you can't hide any chosen Event Cards by keeping them covered until they are activated.

Thanks!

So it's the 2º option (actually I prefered the 1º, but it's not the first time I'm wrong!)