Charles Ellicott Commentary Zechariah 12:5

"And the chieftains of Judah shall say in their heart, The inhabitants of Jerusalem are my strength in Jehovah of hosts their God." — Zechariah 12:5 (ASV)

For shall be, read are. The strength of the fortress of Jerusalem should be the saving of Judah, but that strength would depend on the protection of the Lord of Hosts, their God.