When combat happens

Track 1's first move ended with a unit arriving at a battlefield where no other player's units stood. This track is about the other case: a battlefield where your opponent's units are already waiting.

The start is the same as before. Moving a unit to a battlefield its controller does not control makes that battlefield Contested, as long as it was not Contested already.¹Rule 450. The Destination becomes Contested if it is an Uncontested Battlefield not controlled by the controller of the Unit or Units that moved. Open in the rules browser Every completed move is followed by a cleanup.²Rule 453. When a Move action is complete, perform a Cleanup. Open in the rules browser

A combat is staged

That cleanup is where the two cases part. At each battlefield that Contested was applied to, if units controlled by opposing players are present, the cleanup marks a combat as Staged³Rule 323.9. 7. Mark a Combat as Staged at each Battlefield that Contested was applied to that have Units present controlled by opposing players. Open in the rules browser there. Staged means units of two opposing players share a battlefield, but the steps of combat have not started yet.⁴Rule 461. Combat is considered Staged if there are units controlled by two opposing players at a Battlefield but the Steps of Combat have not been initiated. Open in the rules browser A combat stays staged⁵Rule 323.9.a. The Combat remains Staged at that Battlefield as long as there are Units present from two opposing players there. Open in the rules browser only as long as units from both players are still there, and a staged combat that stops being staged before it opens is never resolved.⁶Rule 461.2. If Staged Combats stop being Staged before the Steps of Combat are initiated, they are not resolved or executed. Open in the rules browser Combat is always between units of exactly two players.⁷Rule 462. Combat can only occur between Units controlled by exactly two players. Open in the rules browser

The same cleanup then decides whether the combat starts. The turn is in a Neutral Open State when no showdown or combat is in progress and nothing is on the chain.⁸Rule 310.1. Neutral Open: There is no Showdown or Combat in progress and no Chain exists. Open in the rules browser If the turn is in that state and a combat is staged, the turn player chooses a battlefield with a staged combat, and combat begins there.⁹Rule 323.13. 10. If the current state is a Neutral Open State and one or more Combats are Staged at Battlefields, the Turn Player chooses one of those Battlefields. Combat begins there. Open in the rules browser The quiet showdown from Track 1 is opened only at a battlefield where no combat is staged.¹⁰Rule 323.12. 9. If the current state is a Neutral Open State and one or more Showdowns are Staged at Battlefields without a Combat staged, the Turn Player chooses one of those Battlefields. A Showdown begins there. Open in the rules browser

A combat can also grow out of a showdown already running. The quiet showdown from Track 1 is what the rules call a non-combat showdown. If another player's units become present at that battlefield while it runs, it turns into the showdown of a combat, a Combat Showdown, in the following cleanup.¹¹Rule 316.8.b.1.a. These Showdowns are called Non-Combat Showdowns. During the proceedings of a Non-Combat Showdown, units controlled by a different player may become present at the Battlefield where the Showdown is ongoing. This will cause the Showdown to become a Combat Showdown in the following cleanup. Open in the rules browser The next lesson is about that showdown. A Standard Move cannot bring those units in, because no unit may take one¹²Rule 144.1.c. This action cannot be performed during a Showdown or Combat. Open in the rules browser while a showdown or combat is in progress, so it takes a card or ability to do it.

Attacker and Defender

Once combat begins, the two sides get names. The Attacker is the player whose units applied the Contested status to the battlefield.¹³Rule 464.2.c.1. The Attacker is the player whose unit(s) applied the Contested status to the Battlefield. Open in the rules browser The Defender is the player who did not.¹⁴Rule 464.2.c.2. The Defender is the player who did not apply the Contested status to the Battlefield. Open in the rules browser The units each of them controls at that battlefield take the same designation as their controller.¹⁵Rule 464.2.c.3. The attacker and defender gain the Attacker or Defender designation now, as appropriate. Units at the Contested Battlefield controlled by the Attacker or Defender gain the Attacker or Defender designation now, as appropriate. Open in the rules browser A unit of either player that arrives later gains its designation in the cleanup after it arrives.¹⁶Rule 464.2.c.3.a. If a Unit controlled by the Attacker or Defender becomes present at this Battlefield after this moment, it will gain the Attacker or Defender designation during the Cleanup phase following the action that caused it to become present, as appropriate for its controller. Open in the rules browser

Notice what the definition is built on. When your own Standard Move made the battlefield Contested, you are the Attacker. The rule does not say the Attacker is the turn player, or the player who controlled the battlefield; it says the player whose units applied Contested, and that is the question to ask.

One thing does not move once combat begins. While a combat (or any showdown) is going on at a battlefield, control of that battlefield cannot change until a step of it says so.¹⁷Rule 190.4.b. While a Combat or Showdown is ongoing at a Battlefield, Control of that Battlefield cannot change until instructed by steps of the Combat or Showdown. Open in the rules browser Outside combat, Track 1's rule holds, and you keep a battlefield for as long as your units are there.¹⁸Rule 190.4.a. If a player controls Units at a Battlefield, outside of Combat, they maintain Control of that Battlefield for as long as they have Units at that Battlefield. Open in the rules browser Inside combat, control waits for the combat, and the step that settles it is in the last lesson of this track.