Mormon 3:9

And now because of this great thing which my people, the Nephites, had done, they began to boast in their own strength, and began to swear before the heavens that they would avenge themselves of the blood of their brethren which had been slain by their enemies.

Because my people, the Nephites, had done this great thing, they began to brag about their own strength. They swore to heaven that they would get revenge for the blood of their brothers who had been killed by their enemies.

Because of these victories, my people started boasting about their own strength. They swore by heaven that they’d get revenge for every Nephite the Lamanites had killed.