Combat damage is divided into two separate steps when any attacking or blocking creature has first strike or double strike (CR 509.1, 510.1). In the first combat damage step, only creatures with first strike or double strike deal their damage. After that damage is processed — including state-based actions like checking lethal damage — a second combat damage step occurs where creatures without first strike (and double strikers again) deal their damage (CR 702.2b, 510.5).
This means a first-strike creature can kill a creature that lacks first strike before that creature ever gets to deal its damage back. If the defending creature dies from first-strike damage, it is removed from combat and will not deal any combat damage in the second step (CR 510.5).
State-based actions are checked between the two steps (CR 704.3). A creature that has received lethal damage is destroyed before the second combat damage step begins, so it deals zero damage in that step.
Example: Your 2/2 with first strike blocks your opponent's 3/3 with no special abilities. In the first combat damage step your 2/2 deals 2 damage to the 3/3 — not yet lethal. The 3/3 then deals 3 damage to your 2/2 in the second step, killing it. But if it were a 3/2 first striker blocking a 2/3, the first striker deals 3 damage (lethal) to the 2/3; state-based actions destroy the 2/3 before the second step, so it never deals its 2 damage back.
If both creatures have first strike (or both lack it), they deal damage simultaneously and can kill each other at the same time (CR 509.1).
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.