打败(dǎ bài) - to defeat, to beat in Mandarin

打败 is a resultative verb: you do the 打 and the other side ends up 败. Because the result belongs to the object, 我们打败了他们 means we beat them, never the reverse, however odd that looks when you translate the characters one at a time.

Use it for contests with a loser: matches, elections, wars, arguments. If you only want to say you won, without naming who you beat, use 赢 instead. 我们赢了 is we won; 我们打败了他们 names the victim. The passive with 被 is common and natural here, as in 他被打败了.

The trap is 打败 against 战胜 and 击败. All three mean defeat; 打败 is the everyday spoken one, 战胜 leans literary and also covers overcoming difficulties, and 击败 is the register sports journalism prefers.