Then they will attack us.

He sent the same order by three messengers, but one of them was in reality a spy of ours, and he came straight to General Grant with it.

We're forewarned, and the trap can't shut down on us, because General Grant means to go at once for Pemberton." Dick understood the situation, which was both critical and thrilling.
Grant was still in the heart of the Confederacy, and its forces were converging fast upon him.

But the grim and silent man, instead of merely trying to escape, intended to strike a blow that would make escape unnecessary.

All the young officers saw the plan and their hearts leaped.
Dick, in the excitement of the day, forgot about the Woodville house and its inmates.