I assume you are referring to act 1, scene 2, as act 1, scene 1 of Macbeth simply features the the witches planning to meet Macbeth as the storm rages around them. In scene 2, we are given much more gratuitous detail of the battle and Macbeth's part in it.

In this scene, a wounded captain details the events of the battle to Duncan, the king of Scotland. It paints Macbeth in a very favorable light to say the least. The battle had raged between the Scottish Generals Macbeth and Banquo against the traitorous MacDonwald and the Norwegian King. The battle seemed to be headed for defeat until Macbeth, against impossible odds, fought through the enemy to kill MacDonwald.

The men rave over the bravery of Macbeth. The captain says that the king of Norway then brought fresh troops to the field, but this frightened the two generals "as sparrows eagles, or the hare the lion." He reports that Macbeth and Banquo fought back with twice as much force and won the day. As the Thane of Cowdor had been a traitor in this conflict, Duncan announces that the thane will be executed, and Macbeth will receive all of his titles.

It is assumed that the battle occurred during Act I, Scene 1. We don’t get the full story of what happens in the battle until Act 1, Scene 2, 3 and 4. The reason Duncan proclaims Macbeth the new Thane of Cawdor is because the former Thane of Cawdor performed some act of treachery or treason during the battle. Duncan promptly orders the Thane of Cawdor’s execution. Duncan says:

No more that Thane of Cawdor shall deceive

Our bosom interest: go pronounce his death,

And with this former title greet Macbeth (1.1.63-65).

Despite the Thane of Cawdor’s treason, Macbeth led the Scottish forces to victory over Macdonwald and the Norwegians.

In Act 1, Scene 4, Malcolm informs Duncan that the former Thane of Cawdor has been executed. Prior to the execution, the Thane confessed and asked forgiveness. Duncan regrets having ordered the execution. But Duncan is comforted that Macbeth, who up to this point has been loyal and a worthy soldier, is taking over Cawdor.