Can you play Recruit the Vanguard during a showdown in a battlefield you're attacking (that you don't control) to trigger Garen's legend ability?

Ruling: No, you cannot play Recruit the Vanguard in a battlefield you are attacking. The card specifies "in your base or in a BF you control," so you can only place recruits in locations you already control, not in a battlefield you're contesting.

Nuances:

• All recruits from Recruit the Vanguard must be placed in the same location (either all in your base or all in one battlefield you control); you cannot split them between multiple locations.