対戦, 対抗, and 対決: opponents, resistance, and a showdown
Read a named opponent, preparation to oppose a strong school, and a final or story showdown without turning preparation into an actual encounter.
対戦 (たいせん): identifying an opponent
「決勝で去年の優勝者と対戦した。」 (“I faced last year's champion in the final.”) Read the relationship with last year's champion as the opponent. The opponent's past title does not establish who won this encounter.
「次の対戦相手の特徴を映像で確認した。」 (“We reviewed video to study our next opponent's characteristics.”) Watching the next opponent's video is preparation. Reviewing the footage does not report that the match has already taken place.
対抗 (たいこう): opposing a target
「強豪校に対抗するため、守備を重点的に練習した。」 (“We focused on defense to compete against the powerhouse school.”) The defensive practice has the purpose of opposing a strong school. The sentence does not report that the teams have already faced each other.
「兄弟は互いに対抗し、同じ大会にそれぞれ作品を出した。」 (“The brothers competed by entering their own works in the same contest.”) The brothers' works provide a different kind of opposition or rivalry. This use is not restricted to a sports match or defensive training.
対決 (たいけつ): a final or narrative showdown
「決勝で二人の好敵手が再び対決した。」 (“The two rivals faced each other again in the final.”) Two rivals face off in a final here. Do not identify them with the champion or video-review participants from separate examples.
「物語は主人公と宿敵の対決で最高潮を迎える。」 (“The story reaches its climax with a showdown between the protagonist and his nemesis.”) Read the confrontation at the story's climax. Preserve the quoted translation, but do not add a gender, an unstated relationship, or a later winner to the Japanese account.
Does defensive practice to oppose a strong school establish that a 対戦 with that school already happened?
Show the answer and why
No. The sentence gives the purpose of the practice, not a completed match or its result. Keep the people and events in the separate examples independent.