Does the beginning of combat step allow players to cast spells 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 507–511). The beginning of combat step (CR 507) occurs before any attackers are chosen, giving both players a window to act first.

During the beginning of combat step, the active player receives priority first, then the nonactive player (CR 116.3). Each player may cast instants, activate abilities, and trigger abilities resolve normally. Only after both players pass priority in succession does the game advance to the declare attackers step (CR 116.4).

This step is strategically important. The defending player can use it to create blockers, tap potential attackers, or otherwise affect the battlefield before the attacker commits to an attack. The attacking player may also wish to buff creatures or set up combat tricks here rather than waiting.

Example: Your opponent has a 2/2 creature untapped. Before they declare attackers, you cast Fog of War (an instant) during the beginning of combat step to tap that creature. Since this happens before the declare attackers step (CR 508), your opponent cannot declare that tapped creature as an attacker, because only untapped creatures may attack (CR 508.1a).

Note: some cards specifically trigger or have abilities that function "at the beginning of combat" — these also go on the stack during this step before attackers are declared, following the same priority rules (CR 507.1).