Alma 51:10

But behold, we shall see that his promise which he made, was rash; nevertheless, he did prepare himself and his armies, to come to battle against the Nephites.

We will see that his promise was foolish. But he got himself and his armies ready to fight against the Nephites.

It was a reckless promise, but he was dead serious. He prepared himself and his armies to attack the Nephites.