Does the beginning of combat step allow you to activate abilities before attackers are declared?

The combat phase is divided into five steps: beginning of combat, declare attackers, declare blockers, combat damage, and end of combat (CR 505–511). The beginning of combat step (CR 507) comes first, and like every step, it includes a round of priority where players may cast spells and activate abilities before anything combat-specific has happened.

During the beginning of combat step, the active player receives priority first (CR 117.3a). Both players may then take actions — activating tap abilities, casting instants, or using other abilities — before the game ever moves to the declare attackers step (CR 508).

This timing is especially important for abilities that affect which creatures can attack or block, or that change power/toughness before attackers are chosen. It also matters for effects that trigger 'at the beginning of combat' — those triggers go on the stack during this step and must resolve before attackers are declared.

Example: Your opponent controls a 2/2 creature. During your beginning of combat step, you activate a -2/-2 ability targeting it. The creature dies before you ever declare attackers, so it never gets to attack this turn.

Note: If you want to respond to an 'attack trigger' or 'when X attacks' trigger, that is too late for the beginning of combat step — those triggers appear during the declare attackers step (CR 508.1). Plan your ability activations in the beginning of combat step if you want to act before any attacker is chosen.