Macbeth calls the day of the battle "foul and fair". what does he mean by this? how can a situation be both foul and fair? (act 1)

Hey there!
What he means by foul and fair is that first of all, war is always foul. People die, and it can end in victory for one side, but they will undoubtedly suffer losses.
However, there's a specific reason for this war. It's a war based on a disagreement or argument, and war was the way it was settled.
A situation can be both foul and fair if it has equal pros and cons, like suffering losses but settling an argument.
Hope this helps!

Thank you so much !!