Jehoash mentions as reason for Amaziah’s declaration of war, the fact that he has become overconfident, but see also 2 Chronicles 25:20. God even uses the words of the ungodly to sometimes warn us. We need to think about human considerations behind which something bigger is hidden: God can use this human thinking and these human actions to fulfill his council (see 2 Kings 14:11).
When we take the expression from Jehoash literally, we have a starting point for a pastoral notion. Jehoash says literally: You have indeed struck down Edom. And your heart has lifted you up (you have become haughty.)
The actual reason to wage war is not the victory, but Jehoash’s own heart. Often people point to all kinds of external causes as reasons for their behaviour (and those reasons can certainly play a role). But their own inner being (character, heart condition, personality) play at least an equally important role and must often be seen as decisive. Jehoash calls on Amaziah to retain his honour and not wage war. In the meantime he has rather humiliated Amaziah with the fable (2 Kings 14:9).
10 You have indeed struck down Edom, and your heart has lifted you up. Be content with your glory, and stay at home, for why should you provoke trouble so that you fall, you and Judah with you?”