Commentators explain that by mentioning His 'controversy' with Judah, the more faithful kingdom, God is sending a stark warning to the northern kingdom of Israel (called 'Jacob' here). If God will not spare the 'green tree' (Judah), how much more certain is the punishment for the 'dry tree' (Israel), whose rebellion is far greater? God's judgment on the less sinful is a promise of judgment on the more sinful.