At the beginning of the play, Macbeth and Banquo are fighting against the forces of the rebel, Macdonwald, who has betrayed king and country to mount an attack on Scotland. The captain who tells Duncan and Malcolm this paints a pretty frightening picture of Macbeth as he fought.
For brave Macbeth (well he deserves that name),
Disdaining Fortune, with his brandished steel,
Which smoked with bloody execution,
Like Valor's minion, carved out his passage
Till he faced the slave;
Which ne'er shook hands, nor bade farewell to him,
Till he unseamed him from the nave to the chops,
And fixed his head upon our battlements. (1.2.18-25)
Macbeth fought valiantly, caring nothing for his own safety, used his sword -- still steaming from the hot blood on its blade -- and cut a path through the fighting men until he reached Macdonwald, the traitor. Macbeth proceeded to split him from his belly to his chin and then place his head on a spike as a warning to any other would-be rebels. Both Banquo and Macbeth then pursued the rebel army, "as sparrows eagles or the hare the lion," as the captain says (1.2.39). So despite their force's relative weakness, Banquo and Macbeth continued to put their lives in danger to be assured of a victory for Scotland and King Duncan.