Ether 8:3

And when he had gained the half of the kingdom, he gave battle unto his father, and he did carry away his father into captivity, and did make them serve in captivity;

When he had half the kingdom, he fought against his father. He took his father prisoner and made him a slave.

Once he controlled half the kingdom, he declared war on his father. He defeated him, took him prisoner, and forced him into slavery.